I've seen that a number of museums that accept the Paris Museum Pass also have timed entries. In looking, for example, at the Pompidou Centre I don't see how to use the pass to reserve a time entry. Is this an issue or can one simply show up at the ticket counter, present the pass, and enter at any time?
At the Pompidou you are only able to visit the permanent collection not the many interesting special exhibits with the Museum Pass. You can select a 'free ticket' reservation for the permanent collection by going to their site on line and choosing ticket for the permanent collection and then a free ticket.. You can I think buy special exhibit tickets when you are there.
Think carefully about buying the PMP. It has no value unless it actually saves you a lot of money because you are doing lots of museums in a short period. It has no other advantages. The few it once had were stripped away a few years ago due to abuse by Chinese tour companies who ran multiple groups through on single museum pass purchases. It never gave line advantage over just buying a ticket in advance and now post COVID since most people have a reserved ticket, lines are longer and there is less line advantage for that.
This page of the Paris Museum Pass website explains how to make advance reservations for the museums which require them if using the pass.
I generally do the pass but I’m going to be honest I don’t think it saves me any money. I bought it for convenience and the ability to pop into small museums that I might not otherwise go in. that might not be worth it to other people. And I will say that with the reservation system it’s harder to make it work in my opinion