I have no qualms about any impositions of any kind.
And I'm always in favor of anything that hacks away at the power of OOC clans and BM's golden-oldies (myself included), and a 1-character-per-continent rule would do that.
That said, this:
One noble per continent? Yeah, congrats on killing most donations to BM. I'd have more nobles than continents I would want to play on. Not a huge fan of AT, Dwilight, or the Colonies.
Is a fair concern, with an easy solution.
Make two-characters-on-a-non-Dwilight-continent possible... for donors (but maybe keep a 1-character-per-realm restriction, though with big alliance blocks that still isn't much of a restriction).
I predict a significant uptick in donations with such a rule, as the benefits to donating would be relatively bigger, and present donors would be relatively unaffected.