No, I'm pretty sure they will not allow an E on a G.
Probably for the same reason they wouldn't allow a B-25J skin on a B-25C model, or probably wouldn't allow a P-51A skin on a P-51B model.
The plane was pretty different. Performance envelope, payload, defensive capabilities, it's enough (IMO) to set it apart from the late model B-17G we have with the chin turret and the Cheyenne tail gun.
Actually, mentioning the performance differences... (Sorry, I'm a B-17 nerd)
Now forgive me, I am typing this from memory while sitting in class so there may be an error or two.
The difference from E to G is incredibly minor. The E, F, and G all have a visibly different nose section (How I know that's an E and not an F) which, really only served to affect aerodynamics. The F and G models have different models of engine which are slightly different, IIRC optimizing the aircraft for higher altitudes. Then, you have the Cheyenne turret and obviously the chin turret.
Airspeed differences are incredibly minor. G's max airspeed is about 10mph higher IIRC. (Keeping in mind the F has the highest cruising speed by 10mph).
Payload is identical. The lifting capacity by weight in the B-17 is greater than the bomb-carrying capability due to the dimensions and layout of the bomb bay.
Defensive capabilities are remarkably similar. The tailgun (Stinger configuration IIRC) was no AS effective in the B-17E, but she still had two .50s IIRC. The chin turret is lacking, but the cheek turrets are there.