How do you prevent it being mixed with other coins?
Yes I thought of the same thing yesterday.
It makes the whole a little less elegant.
I suppose you should check if more than the master BTC part is the product of the transaction, than it's "destroyed".
For example:
0.1 + 0.0001 (fee) = 0.1 Valid
0.1 + 1.0 + 0.0001 (fee) = Invalid