Generally, no, that would not be allowable as it (deposit timing) would be a discriminatory BRF. Maybe 6% owner PS and 0% NHCE PS deposited throughout the year would be OK but as you say, there's no guarantee it would pass testing. If more NHCE PS is then required, would that create retroactive BRF discrimination? Maybe, probably would not know unless and until audited, if ever. Certainly going more than 6% is not a good idea. How important is getting that extra 6% in sooner compared to the possible risk? You can communicate the issues and, where there may be compliance ambiguity, the owner can decide how to proceed and accept any risk (but get it in writing).
Another concern may be if PS provision has any conditions for entitlement in the document.