just thinking
if you pulled one spark plug in each of the 3 cylinders and a compression gauge in one and hooked it to a good charger on crank assist that might crank that motor fast enough to disable the decompressors. probably necessary to disconnect fuel or wide open throttle so motor won't start.
or pull the wire on cylinder are checking.
I'm sure you could- you'll see once you get to the cams. I'd try @TURBO200R4 suggestion first. They probably disable after after a revolution or two, I would think.
No ones asked, so, what's going on? Why are you concerned with the compression?