You still retain all of the cooling effects of the DI even though you are adding port fuel, and you can stage the system if you wish as well.
What is your response on my comment about measuring rail pressure? What have you found?
There are many cars that use both today, including the Lexus IS-F.
Save yourself the trouble, and add port fuel on top of it, you'll be just fine. Direct injection works more like port fueling at high RPM's anyways due to the volume and injection duration. It doesn't atomize quite the same as low rpm. You'll notice a lot of DI cars running 14.5 A/F usually up to around 5000 rpm, after that A/F ratio will drop significantly to around normal port fuel levels.
Seriously. Go on ebaymotors, pick up a LY7 Lower intake manifold, Upper intake manifold, and fuel rail. Heck you could even use stock injectors if you want to start - you'll get them for a song. The issue will be in mating up a simple stand-alone for the fuel trigger and injector control. Problems solved.