Changing reward budgets per post could be problematic.

First, there is the issue of user education. Currently, users know that they get paid for posting and curating. The math is publicly available in the source code, but doesn't need to be advertised to everyone for them to understand how to use Steem. If we added such an option then it would need to be reflected in the UI so users knew how much of the rewards they could earn.

Second, while this might start as a way for different interfaces to incentivize different content, it would not stay that way for long. Savvy users would see that a configurable percentage creates a market for potential voters. If there are identical posts, one with 25% curation rewards and the other with 75%, which one will most people vote on? The answer is both, but at some ratio defined by the curation rewards curve.

This would turn curation into a market. How much of your rewards are you willing to give to curators for votes? This a slight change from the intended mentality today that curation is budgeted for rather than authors paying curators for their votes. The difference is subtle, but the perception of ownership over tokens is a powerful one. This is why many users get upset when they are down voted. The rewards are not yours until payout. But that doesn't prevent users from feeling like the rewards are theirs as soon as they receive an upvote.