Unidan could just create an account on multiple instances and vote for his posts/comments with all these accounts. That way his content would gain more attention than those of sincere users.
In case of malicious bots (like those annoying bootleg bots on reddit), it might even be profitable for them to create their own instance(s) just for that purpose.
Is there a mechanism to prevent that? (other than user/instance banning and the introduction question on user creation)
Nice idea.
Global and local votes is an interesting idea, reflective of echo chamber vs general consensus.
Not sure how effective it would be in addressing the point raised by OP, but it might have value in its own right.