RE: getting the mast extension shim out of the mast
If you know any one with one of the older extensions with aq spring loaded "keeper pin" for the shim, see if you can borrow it long enough to stick it in and retrieve your shim.
I like the idea of a "half round" shim extractor tool. It should not hurt the inside of your mast at all as it will have alot of surface area.
My guess would be (from digging out the same shims many times, years ago) tha the shim will come loose pretty easily (especially the plastic ones) and can be removed from your mast without damage to either the shim or the mast.
You could make the tool from a bl;ock of wood cut to the inside diameter of the mast.
Give it a slightly "beveled" sharp edge away from the "downside" and it should go up and hook the top end of the shim pretty easily.
As suggested, once you get the shim out, split it, and you should have no further problems unless something "sticky" is on the outside of the shim or the inside of the mast.
You could actually get a 2 point almost full contact "hook" if you shaped your hard wood shim extractor so it's a little "cammed" and will go up inside the shim with a little angle to it, and the as you rotate it the 2 outer points would engage to top of the shim.
Put the wooden hook on a short piece of threaded rod, with a nut and washer set up to take the tapping. Find something heavy (like a lead sinker) to put above the nut and washer and you'll have an instant "slide hammer".
Hope this helps,