I think for the most part delegates can be trusted to provably burn any surplus pay and thus manually do what you want to add a lot of complexity to the protocol to do manually.
A trusted delegate campaigns for something like 2x what they need and promises to burn anything over $x USD worth of pay. They can then have a predictable pay rate and prove their trustworthiness by doing regular burning of any surpluses.
Well then they would need to have some standard way of publicly burning BTS, and I would really want to see something like bitsharesblocks.com track this and estimate the USD value of the net funds each delegate is keeping so that stakeholders have an easy way of keeping track of whether the delegates are keeping their promise. Otherwise you are making the voters job much harder, and we already know how apathetic voters already are.
Also, the other benefit of the method I proposed is that one person can earn more than the maximum 1562224 BTS per year (about $26,000 at the current market cap) without needing to take over multiple delegate spots (which is good for decentralization). The only requirement is that the total income of all 101 delegates valued in BTS is less than 5050 BTS per every 16.8 minutes. So again using current market cap, this means that we could have 76 regular delegates getting paid $1000 per year to run the machines as well as 25 delegates getting paid $102,000 per year. Under the current proposed system, achieving this would require each of the 25 people getting paid six-figure salaries to have 4 delegates (for a total of 100 delegates) leaving only 1 delegate to get paid the low $1000 per year. So you centralize the power from 101 unique individuals running the DPOS consensus (in my proposed system) to just 26 individuals (in the current proposed system).
I'm sorry that every other post of mine is just putting more work on your already full plates. But I have reduced this proposal down from my ideal original complicated system to the simplest possible system (and I am pretty sure the code changes are very minimal) that I can come up with that still achieves the desired effect of letting voters set the salaries of business delegates in USD without compromising DPOS decentralization too much. I think this is highly beneficial to have especially as we are becoming more dependent on delegates to fund the development of BitShares rather than I3.