I like your idea. Even when I raise the mast by just lifting it up I put a lot of upward force on the mast base.
where the bottom of the compression post sets on the liner, between the liner and the keel trunk, there was supposed to be a pad built up in there...
depending on the size of the gap, it was folded up resin saturated cloth, or if it was a really close fit, maybe nothing at all.
if you have nothing at all in there, its is still possible to take a bottle jack and jack the liner up a bit away from the keel trunk and put some bedding in there.... it will add better overall support.
figure out about how much gap you have, then just jack it apart and lay several layers of resin coated cloth in there to build it up thick enough so when you let the liner down onto it, it makes firm contact and beds in.....
then commence to shimming the post....
I think with any light built boat, everything beds in and finds its place over the years with the slack that is either available, or is created by the stretching and working of the fiberglass.... once this process has settled, the hull can be, and should be, re-tensioned properly by adding a shim to the compression post.
before jacking the cabin top you need to remember to take the bulkheads loose from the roof, so you dont rip the screws out as the roof goes up.... and you may have to move or shim them when you replace the screws IF you get the compression post too long when you shim it....
and although there would be very little reason to jack more than an inch or so, theoretically, for structural design, you cant really get the compression post too long, as when the cabintop is farther away from the hull, (more space between the two) it makes a stronger shape.... you are only limited by the existing interior fittings and the cut of the existing rigging..... DISCLAIMER; you must understand though that you cannot jack it so much that you will ever be able to get standing headroom inside the cabin:cry:, but up to 2.5 maybe 3 inches higher is do-able, but only if you were going to refit the entire boat.
I jacked my mac21 cabin top 2 inches when I was working on the compression post and tabernacle... but let it down with only a half inch worth of shims added to the post. the broader area of the 25 would allow more.
after doing so, you can even tell by walking on the cabintop WITHOUT the mast in it, that it is a much tighter hull....