I doubt this is the best place to ask such a question since few here know the guts of Android well enough to help you. Perhaps it would be better to ask on the NITDroid forums or the xda-developers forums.

I expect the drivers you need for hardware acceleration have some dependencies on the kernel and the hardware is probably not quite similar enough to just drop the Milestone kernel on the N900 and have it work. So, to do what you want to do you probably need get the necessary parts from the Milestone kernel into the N900s kernel. The Milestone's kernel should be open source and N900's certainly is (though it may be better using the kernel from the N900 version of Meego which is newer than the Maemo kernel) so it should be possible, but not simple.

I could be on the wrong track here about needing bits from the Milestone kernel to get it working, since I'm not an expert on these things, but I don't think I am.