...can it be done in a microwave?

And would CaSO4 (gamma-anhydrite) be preferable over MgSO4 (mono-hydrate) as a drying agent for acetone or not?

Thanks

Yes, you can dehydrate either in a microwave.

Both will work to dehydrate acetone. They behave somewhat differently when they absorb water. CaSO4 should remain solid while reverting to the hydrated form. MgSO4 will also, but if there is a lot of water it may turn into a syrup. Although the syrup will form the bottom layer from which your solvent can be easily decanted. Most often there is not eneough water present in acetone to do this.

In the past sister used to toss a few clumps of MgSO4, directly into the fresh can of acetone so that any atmospheric water that gets into the acetone would be absorbed, making the acetone ready to use off the shelf at any time

Thanks...

Do you have an idea as to how long they should be microwaved or any other advice regarding this procedure?

CaSO4 is considered better than MgSO4 for dehydrating solvents for a couple of reasons. In the case of acetone, it has much less of a Lewis acid catalytic effect which with MgSO4 can cause a condensation reaction producing mesityl oxide and water, which is a little counter-productive, but if you're not letting the acetone stand on the MgSO4 for too long it's not that much of a concern. Also CaSO4 is more efficient at drying even though you have to use more of it, and it doesn't liquefy if there's lots of water in the solvent.

I think for the intended purposes here, there's pretty much an equal preference either way.

When MgSO4 is dehydrated is changes from a clear/transparent cystalline structure to an entirely opaque white chalky structure which will fuse together and be quite brittle.

It has been a while, but if memory serves it actually melts in the oven then forms the opaque white fused look when it is fully dehydrated. Then you need to pulverize it to get it to a powder form again.

I'll try a small amount in the microwave - fingers crossed...

I like mgso4 bacause of the easy calculations; fully hydrated epsom salts lose half their weight when baked in the oven and it follows that they absorb their weight in water.
e.g. 100 g of epsom salts dehydrate to roughly 50g of anhydrous mgso4; and these 50g can absorb up to ~50g (or 50ml, same thing) of water.