Would it be possible to prevent upvoting of questions when they reach an (equalized) rating of +10? Since you get only five votes on each proposal, why would you waste them on bringing a question to 11 when only 10 are needed?
No, you want to give the community a chance to vote for all questions, both up and down. The score needs to reflect (budding) community opinion.
By locking a question to a score of max 10, you remove that option. Downvoters would gain an unfair advantage, as their downvote would suddenly weigh more than the upvotes.
The voting process defines the community, locking the score to 10 would also remove the ability to find consensus as to what makes a great question versus one that is merely a nice one. They'd all look the same, stuck at +10.
Martijn's post pretty much covers the pragmatic considerations, but you also have to consider the purpose of voting on questions. It's to show that is a sufficient breadth of high-quality questions to start a site. If (hypothetically) a proposal had only one decent question, you wouldn't want to simply shuffle people's votes the the next "least-objectionable" question in the line up.
We need 40 questions you would judge as "high-quality"; not just 400 votes on… whatever.
Getting a proposal through Area 51 is tough going. It takes a lot of support, engagement, and quality contribution — That is by design. Shuffling the votes around to optimize the process into a production line somewhat defeats the purpose of Area 51. Every vote counts and no one should be told their vote is being wasted. In our evaluation, such efforts do not bode well for the proposal.
I wonder about yet another option:
Votes on questions that have score of 10 or more don't count towards the 5 upvotes limit.
That way we get the best of both worlds and more:
People won't feel they "lose/waste" their votes on questions that are already above 10. They can still differentiate between best questions and ones "marginally fitting". We'd finally see which questions are the embodiment of spirit of given site, instead of uniform row of +10, +10, +10... and simultaneously "far pages" would get more attention. (note this might lower the threshold for a site to reach commitment phase.)
Also, authors of good questions wouldn't lose deserved karma. I often move my vote from a "+11" to a "+9" question. Not that the "+9" is better than the one at "+11" but the firm threshold is what counts for the system - even though the +11 didn't deserve losing a point.
How to implement that remains to be decided - e.g. whether voting to +11 leaves no impact on the current voter's vote pool ("stack"), or costs the voter one of the 5 questions but returns a vote to the pool of votes of the one who cast the first vote on that question, allowing them to upvote yet another question upon revisit ("fifo").