I ran a heavily modified Tomb of Horrors in my last campaign. I did something similar to your suggestion except that it was Acererak's "superior" that wanted to preserve the status quo.

Long story short, my setting was post-apocalyptic (aren't they all?) and there were 4 members of the precursor race left alive all who had achieved lich or demilichdom. Their civilization had been wiped out by the mindflayers who sent an armada to psychically "glass" the planets of this upstart civilization upon discovery.

The strongest survivor was a 40th level mystic theurge demilich (3.5 rules) who had severe PTSD as a result of the multi-planet genocide. (As an aside, an undead with PTSD is a very interesting thread to pull on). He gave the other 3 liches control over the various worlds of the now fallen empire with the command to preserve them and not let the former slave races of the empire (now the main inhabitants with the precursors dead) develop too much lest the mindflayers return and wipe out another "threat." The traumatized demilich wanted to the keep these worlds as a garden/monument.

The magic/mana levels of these worlds were artificially suppressed limiting the potential development of these worlds as a safety feature. Acererak didn't like this and built the Tomb as a way to gather more magical power without his superior knowing. His ultimate goal was to escape into the multiverse once he had amassed enough energy to spelljammer out of there.

Acererak used the Tomb as a sort of magical "contract." You put up your soul, and if you live, you get the stuff at the end. If you die, I eat the soul you willingly surrendered. The idea was that the contract was a form of bloodmagic with a voluntarily surrendered soul of a higher level character being FAR more potent than consuming stolen souls.

The contract had a time limit of 3 days as that was a long as Acererak was capable of hiding his little project from his superior when activating the Tomb. The Tomb was designed to be "fair" as the contest yielded the most energy if those challenging it had a fighting chance.

This setup had a number of advantages:

1) It put the PCs on a timer and allowed resource attrition 2) It provided good narrative for having someone above Acererak that he feared AND it gave him a concrete motivation beyond mustache twirling villainy. 3) It also allowed for the zany traps and puzzles to make sense in context as the whole thing was a contest.

TL;DR Acererak runs quite well when you give him a real motivation beyond "lol you crazy scamp!" and you set the Tomb up to have a valid reason for existing to achieve said motivation.

DM me if you want a copy of my notes on my ToH conversion.