What do you think about a “$-box” instead of “time-box” for this fee discounts? The DAO would have to reimburse solvers for the reduced fees they will be receiving for those orders (otherwise they would have a reduced incentive to actually include these discounted orders in settlements).
This way you would change the proposal to something along the lines of “allocate X$ for additional fee subsidies for COW holders/lockers with the following rules…”. The proposal can then be renewed in order to continue the fee subsidy.
I guess some of the details will also depend on CIP-2: Solver Rewards as this defines how solvers are rewarded and reimbursed for their operational costs (i.e. gas costs).