Can you give some sample scenarios?
The current Dash collateralized is 37.5%. With your idea, if no proposals were funded, the 10% remainder would be distributed in the form of 3.75% to masternodes as extra reward, and 6.25% would get burned/non-created. Is that what you mean?
In short, yes. But if we went down that road it might also be better to weight one side of the allocation because allocation to masternodes benefits them, but the allocation to the burned/not-created option actually benefits both the non-masternodes and the masternodes again, so masternode interests essentially get double counted. But you can actually make a number of different arguments as to how much the weighting should be, depending on how much you think the masternodes are able to skew things in their own interests when allocating non-mno dash inflation, how much the non-mno collateral holders benefit from appropriate mno investments, and how those two categories should be balanced against each other. The simplest and probably the least subjective way to do it is the one in the example you described.
Your example above could be said to have the formula:
mno_payout = collateral_fraction * treasury_fraction;
burn = treasury_fraction - mno_payout;
So if 50% of dash coins are locked in masternodes one month, and no proposals pass, that would be mean that the 10% treasury gets allocated:
Extra MNO Payout = 5%
Total Burned = 5%
Then if in the next month market conditions change and 60% of coins are now locked in masternodes, then the amount of other people's money that MNOs could be spending is lower, they are "spending" or not "spending" more of their own money, and so the formula automatically adjusts the allocation to account for this:
Extra MNO Payout = 6%
Total Burned = 4%
But you could also put in an extra term to remove the double counting of masternode or non-masternode interests as:
mno_payout = collateral_fraction * treasury_fraction - weighting;
burn = treasury_fraction - mno_payout;
So for the first example if: weighting = ((treasury_fraction - collateral_fraction * treasury_fraction) * collateral_fraction);
Extra MNO Payout = 2.5%
Total Burned = 7.5%