I stumbled across this thread wondering the same thing, and thought I'd reply.
I'm obsessed with romancing Cullen, I do wish it was more... involved though. I haven't heard any companion banter yet other than the war table banter, which was cute. I'll have to see if I can get the Dorian or Varric ones.
The Serra one should be available to anyone romancing Cullen who asks her opinion about the advisors on the Inquisition. She says something like, "And your Cullen-Wullen? Cully-Wully? Lots of men under him. Needs a women over him -- because, positions." Haha probably the best thing I've heard her say as I personally don't care for her character that much. :/
I did have something to add though, it's not exactly companion banter, but when you're at the Winter Palace, if you're walking by Cullen, he's always fending off lady admirers... and at one point you can hear him say something along the lines of, "No, I'm not married, yet, but I am taken. What! Did you just... grab my butt?!" And then a lady giggles.
On the other side of the room another group of ladies is talking about him too, someone asks about the attractive inquisition officer, and another woman responds, "Don't waste your time, he refuses to dance with anyone, even Dutchess so-and-so." I thought those little ambient comments along with Cullen's romance conversation with the Inquisitor made that whole Winter Palace scene with him even more special.
Does anyone know if there's romance banter with Cullen when you do his bigger personal quest? Unfortunately I sided with the Templars, and I guess his quest is only available if you side with the mages. I'll have to do another playthrough.
On a side note, does anyone else wish that you could play as your companions and hook them up with each-other? I want to create a Varric/Cassandra romance so bad lol. I've heard that Iron Bull and Dorian will flirt with each other if you don't romance either of them (which I haven't) but I haven't heard any flirting between them when I have them both in my party, so we'll see.
*edit* I've now had Dorian and Iron Bull in my party for a few missions and they definitely have worked up to flirting now. Too funny!