I agree, using reversing entries in the current month would solve the problem, Assuming no-one ever voids a paycheque.
I prefer your suggestion of an archive snapshot at the time of calculation. Something like the T4SUM in nv1 saved by the date or ck# as part of the filename.
The purpose of the request is when Rev Canada does an assessment, they always ask for the details of the remitted amounts. The snapshot archive would provide that exact details of each cheque. Where cheques were voided, they would have to be included in the next archive listing.