Example of Garbage/gas collection for 0x

I considered garbage collection (gas collection) for 0x, these can be generalized to other contracts. Here are my thoughts:

Garbage collection can be used to subsidize certain parties or functionality, by freeing up data and allowing functions to have a net negative gas fee. Pseudocode example:

free(Order[] orders) {
order for each orders
require(order.expiresAt < now)
delete transactions [keccak(order)]
Profit: 9000 gas per order.

For 0x it might be interesting to allow the relayer to call free. The average fee per executed order is around 90000 gas. Gas collection Offers a 10% discount as a subsidy for relayers.

Are there other examples of projects using garbage collection as a subsidy?