Vanilla BG2 also did this kind of balancing in one instance, in the beginning fight of ToB where the number of enemies showing up depends on the MC's level and party strength. I don't get why it has to be much more complicated than that. I have modded and balanced my own game in the past; I didn't find it that big of a deal, trying to tweak a fight for decent balance based on number of party members (or overall party strength). The AI was all there; all I needed to do was adding one or two bad guys, tweaking their levels, adding/removing a couple of abilities, positioning them, then playing test a couple times. Done. I actually found it really fun. Doing this for every single encounter in the game may take some time but I don't think it's that bad. If adding one or two bad guys and tweaking their stats a little bit would break the entire encounter then we have a bigger problem here...
"We make our choices and take what comes and the rest is void."