how does it satisfy 401(a)(4)?
you would have to run the testing. I imagine on an allocation basis you would fail testing as HCEs general have larger account balance so would receive a larger % of pay by the time you divide contribution by compensation.
so now you test on an accrual basis, but that also requires you pass the gateway minimum
as for the document, is it possible to allocate a contribution based on balances? if each employee is in there own rate group, then there is certainly nothing to prevent you from allocating a contribution which, for all practicality is based on account balances, but again, you will be doing crosstesting, gateway minimums, etc.