I have a Trek 1000 that takes a standard 27.2 seat post. Would it be unwise to install a 26.6 from my other bike onto this one?

yes. I will let others elaborate.

Putting a 26.6 in an aluminum Trek would be equivalent to placing your frame in the recycling bin. Seriously, if you tried to crank the bolt down enough to even begin to clamp down, you'd break the tab off. If you need to switch out the seatpost for some reason, buy another 27.2mm post.

You could shim it. Use drink cans or a prefabed shim from an LBS.

It is best to use the RIGHT part for the job.