I managed to fail this during my second/alternative play-through even though one of my main goals was to see this through, for two separate reasons.
A.) First reason:
I always pick basically the same character type for all my first RPG playthroughs. They are supposed to be the closest thing to me, had I lived in that world (and I always carry those over to the next games, regardless if I do a second playthrough with alternative player characters and choices) which is in case of DA games and Thedas: a human noble male rogue (with about 65% diplomacy/kindness, 25% humor and 10% brutality/treachery/arrogance).
Even though he was a heterosexual man (as inherited from me) and I also assumed Solas as such, he started to considered Solas as a very close friend, almost to the extents of a platonic-romance after they met in their dreams (for me, it was probably the most intimate moment in the whole DAI game, even if for the lack of any better). After all, I am heterosexual but not homophobic, so it was fine for my "as close to me as possible projection but not strictly me" character.
And this whole thing got further justified when Solas turned out to be "sort-of a God", thus some level and kind of "affection" could be considered perfectly natural. I really loved how that played out. It was like I never knew but I still felt how he was "something more". It's like I predicted it by feeling it in his presence what he is, I just couldn't comprehend it better without more data...
Sidenote: Not because of this but for a general lack of interest in all the potential candidates, he didn't have any sexual relationships. It was a little disappointing because I found the casual sex opportunities and the romances to be really great in DAO and ME1 and somewhat decent in DAII and ME2/3, so it was unexpected to find nobody from a bigger palette...
B.) Second reason:
After I finished my first playthrough, I started to delve into the SPOILERs on the DA WiKi to see what I potentially missed and thus what I should make up for in a potential second playthroug or watch on youtube (or at least fully read through on the WiKi).
I stopped reading the corresponding page as soon as I realized that there is a notable amount of lore I could only hear if I romance Solas as a female dalish elf.
So, after I concluded that a second playthrough is justified and after I added up A+B, I wished to see (among other things which seemed to be conveniently compatible with a stereotype female dalish elf: "up to 99.9% arrogace") how that romance with Solas would go instead of just watching it on youtube. I tried to make sure I won't read too much spoilers and I tried to do my second playthrough as if I played for the first time (I tried to ignored my hindsight as much as I could...). Thus, I never flirted with Solas until after that dream-Heaven scene because that's where I first realized he is a really interesting character (up to that point, I preferred Vivienne as the mage companion even though I considered her merely a decor for the outsiders, I actually hated and would gladly sacrificed her
).
Although my new female character was a "male elf oriented but omnisexual" dalish elf (because most of the elfs I came across or heard of in Thedas were usually like that: they often preferred but were not limited to their own race and the opposite sex --- at least based on my own accidental joke-flirting with Zevran [which eventually resulted in massive disappointment from him when I stepped back], my bedtalk with Merrill [she managed to be cute about how she "allowed herself to have sex with a human" in a world where elvs in general are more or less oppressed by humans :D] and stories from Fenris + codex pages and random rumors [including Celene and Briala], and the list could go on...), so she shamelessly flirted with almost everyone (except Harding) regardless of race or sex, I was careful not to initiate an actual romance with anybody else. I limited the flirting to one or two occasions with anyone I expected to bite, except with Cassandra because that was very funny and I thought it could be even more amusing to see her struggling with how to turn this thing down (or might even fall for it, however unlikely that seemed).
And now she is about to face Cory for the last time but she had zero active relationship so far. I think I had my last possible chat with Solas (excluding the DLC contents but I doubt those could make a difference with this and I planned to play those after Cory).
So, I read more spoilers on the DA WiKi and finally Google led me here. It seems like I made two mistakes:
- I should have started trying to romance Solas sooner, probably much sooner (even if it wouldn't feel natural because he is really a man with hidden values, not someone who looks interesting after the very fist moment you idly speak with him...).
- I shouldn't have a half-active joke-romance with Cassandra. I think her refusal scene was already scheduled (just not yet triggered) when I could have my last chance with Solas (during and/or right after his personal quest).
Although I did have some chances to use the romance dialog option with Solas a few times after the dream scene (once or twice, I think). Yet, it seemingly wasn't enough and it completely stopped at some point and I was just way too patient to realize it's gone (up to the point when I realized it's game-over...).
Ah, it's amusing to admit as a man that I don't know enough about man. 
I figured Solas should be taken easy and slow (which I would consider ideal plot-wise >> see the reasoning above). Instead it seems like he should be nagged pretty hard..., well..., just like me, LOL. 
Ah well, off to Youtube for those exclusive lore pieces, I guess (I am not sure I even want to kill Cory anymore, I think I did everything else I wished and this world-state were always to be tossed away after completion anyway, regardless of the Solas-romance...). 