Where did they state specifically, that one of the 8 romances would be a non-advisor/non-companion character??
If there are simply some "romance-ish" non-inq party characters, that doesn't mean they are being included in the 8 romances. They could be like Sha'ira in ME1, Kelly in ME2 (not ME3 though, since she does grant the Paramour cheevo), or having a fling/flirt with Zev and Tallis in DA2. IF that is the case, and those type of "romances" are not being included in the 8, then Solas has a very good chance of being an LI.
Let's use subtraction....sorta.

12 = Number of confirmed companions and advisors
2 = Number of confirmed companions/advisors that are NOT romancable (Leliana and Cole)
5 = Number of confirmed companions/advisors that WILL be romancable (Cass, Cullen, Dorian, Sera, Josephine)
5 = Number of unconfirmed companions/advisors that might be romancable (Solas, Varric, IB, Vivienne, Blackwall)
Of those last 5, who would be most likely to not be a romance. My guess is Varric and Blackwall. I wish I had a link for this little bit of info I remember reading, so a disclaimer on the following that I do NOT currently have a link to the following tidbits.
Varric = I recall Mary Kirby bringing up the idea, that Varric and Bianca's story was not over yet. Does that guarantee anything? No. But, it makes it more plausible that they will have reason to keep him unattached for this very reason.
Blackwall = I know that BW does like a couple of "angsty" romances, but Blackwall is a warden most likely not far from his calling. This being established by some notes written by him(??) that somewhat dates his time with the Wardens. Again, sorry for lack of links. This puts him more likely in the no romance list, than not. Of course, it is NO guarantee.
That would mean that out of the 12 companion/advisors, Leliana, Cole, Varric, and Blackwall would not make the list. We would have our 8: Cass, Cullen, Dorian, Sera, Josephine, IB, Vivienne, ANNNNND Solas!!
Sound good?? Make it so, BW.