Is it possible to use IAv5 with Baldur's gate Trilogy BGT? Or is it only possible with a BG2/TOB install?

The safest bet is that they are not compatible. There was no specific work done in order to ensure such compatibility, and IA was always developed for Soa+ToB. All in all, you would probably be better off by playing BG1 separately from IA and importing your character into IA afterwards. Although I am not sure if such an import is a safe course of action (Sikret might clarify that bit), but I think I red somewhere about BG Tutu characters being imported to play IA.