I'm not saying that it wouldn't work, just that it's not good practice. I understand that your case is a particular one being it's a race car.
The problem is that no matter how good your MAP signal is, not having a manifold reference for the FPR means that your VE table will be delivering differing amounts of fuel even if the VE table values are equal. Not having the reference makes tuning more difficult and reduces your resolution in high-vacuum areas of the VE table. Of course it can be done, as you have.
I just disagree with your premise that "most EFI installs want constant pressure". Most EFI installs definitely have MAP-referenced FPRs for the reasons above, especially for street-driven cars as is the case for the OP. I'm including OEMs here as well, doesn't matter whether they're ITB or not.