Any ideas on how to remove it, without damaging the suppressor, or booster? My plan was to tighten something in a vise, and turn the tube by hand, but I can't find anything that fits the end cap well as the booster.

I would put the booster in a vise and use it as the tool, then I'd press that tight into the boost and turn it by hand.
I have a barrel vise thing that I would use to hold it as it's round and wouldn't crush the booster (though the booster is steel, so no real worries there).
If that didn't hold it tight enough, I would use a thin piece of steel through the booster slots and hold that steel in the vise.
I would also use a penetrating oil of some kind on the cap to try to loosen things up.

Thanks Chris. I was thinking about doing the second thing you suggested, but wasn't sure how "fragile" a booster is. This is my first pistol can, so I was a little worried about damaging things. I'll give it a shot, and see how things turn out. At least the end cap, and booster are replaceable.

The booster is TOUGH. You'll see. The problem will be if it slips in the endcap. It'll mar up the endcap and round off edges. So, press hard and don't let it slip.
As you state, however, the booster and end caps are replaceable. If it's REALLY stuck, call/email SilCo and they will fix it.

If you have a LH metric piston, just attach it to a barrel and use the entire gun as leverage.
You can exert more consistent force that way. And less chance of slippage.

Loosening the piston on the barrel is easier. Slip a steel bar in between the slots.
Don't worry the pistons are stainless steel.