FWIW, and I don't claim expertise on this, but I think it is worth noting that Revenue Ruling 86-142 was for a DB plan. I think the scenario is different for a DC plan. I think the RR is saying that the payments for expenses are indeed deductible, but they are contributions. In a DB situation, that's sort of a "who cares" as long as you're not bumping against limits. In a DC scenario, if they are contributions, then they have to be allocated.
Can't say I read every word so I might be off base but that's how I see it. Prior threads indicate they are contributions, but there are no cites that I know of.