Any proscription on contributing "matching" contributions to a governmental 401(a) profit sharing plan based on participant deferrals to a governmental 457(B) plan? This would be similar to matching 403(B) deferrals in a 401(a) plan.

I know that the 401(m) regulations say that the contribution is not a matching contribution if the "match" is based on the 457 deferral. But I cannot find a reason for disqualification. Government 401(a) plans don't need 401(m) to be able to contribute different amounts to different participants.

I would agree that this technique is permissible. Indeed, I recently did an outline in which, among other things, I described this technique, and the reasons for using it. (See Section III of the outline.)