Beyond driver and performance issues, the E7's display is only 640x360. Even if the closed source portions of Maemo 5 otherwise accept the E7's hardware, likely they will be broken by the display. The proprietary portions of Fremantle include enough hard coded assumptions about the N900's display to make system-wide portrait mode on the N900 unobtainable, after all.

I'll go a step further and hazard a guess that many of the existing apps in Extras would also dislike the reduced screen resolution of the E7. So, more breakage. More porting. More writing replacements. In the end, at best you'd wind up with a function-alike of Maemo 5 with a reduced app catalog.