I believe the main concept behind the uppipe installation point is ease of installation, giving the alky enough time to hopefully vaproize completely to remove as much heat from the air as possible and allow even distribution to all cylinders. Moving the point behind the throttlebody would likely change the design performance in all these areas. The propane kits being sold are also located in the pre throttlebody areas.
HTH
Functionally, I don't think it matters, but I think your biggest problem will be siphoning of the alcohol out of the tank due to the vacuum in the manifold. Upstream of the throttle plate you won't have any (certainly not as much) vacuum.