can I use a mercruiser bravo inner plate on a neutral imco extension box?

Yes
I've done a few that way when I couldn't source a heavy duty inner plate

And cut all the steering stuff off.gives a cleaner look

One thing to keep in mind is that the IMCO inner transom plate is much thicker then the Merc one. There is quite a bit of leverage on the transom with a box installed, so having the thicker one isn't a bad idea. Just a thought.
Eddie

we ran a standard merc inner on a 33' sutphen that regularly ran in the ocean and topped out at 93mph.never a problem,but I wish I could have gotten him the heavy duty ones like I run.

I'm not saying that you can't do it. I'm just saying that there is a ton of load on it and there's a reason that IMCO supplies an HD inner transom plate with the box. That's all.
Eddie