I wouldn't know, I seldom read het, overtones or not. Though yeah, Babs/Dinah's pretty scarce. Which is insane. Everyone knows they're stupid in love, yes? It's one of the most widely "yeah, I totally see that"-acknowledged non-canon ships I've seen in femslash. So why are there not more stories? (Not that I'm one to talk, with my ten or twelve WIPs that will never, ever get P'd to completion, but still.)
I mean, there could be a more emotionally plausible threesome, but the fic would be longer (unnecessarily so, IMHO) and once you chuck the spontaneity and kinkiness out the window, I don't think it would be as hot.
Heh. You're such a guy.
Shenanigans ensue.
Depending how far you take that from canon, you could make a pretty interesting commentary on Dinah with that kind of AU. Jealousy has been a major problem for Dinah in the past - sharing does not come naturally to her. Was it different with Hal because Hal was a guy? If so, how well does she tolerate sharing Babs with Dick in the long-term? If Hal is different because she subconsciously devalues queer relationships in favor of het ones ("Ollie slept with someone else, but it wasn't real sex, it doesn't count"), she's really not going to take her girlfriend having a guyfriend well. If Hal is different because she's thinking in terms of variety, though ("Ollie slept with someone else, but it was a guy, and a boyfriend provides different things than a girlfriend so I don't need to feel threatened"), then Dinah will be fine and Babs is going to be the real sticking point.