It's apparently already playable in ScummVM, but it's such a pain to prepare the necessary files from the original discs that I can't express exactly how glad I am that the GOG release exists.
If you *do* decide to get your hands on the original Mac discs, here's a "how-to" guide that includes which files you need to copy in order to play 'Pegasus Prime' in ScummVM: http://wiki.scummvm.org/index.php/HOWTO-Extract_Pegasus_Prime
I looked into the procedure to play 'Pegasus Prime' on my Windows 7 PC and it involves copying (what looks and feels like) hundreds of files into a certain file structure with some files being extracted as files and some as Mac binaries. It looks as though the procedure is a bit easier in Linux (since there seems to be a script some kind soul wrote to do a lot of the heavy lifting) but I would still imagine it might be a bit of a pain in the you-know-what.
EDIT: I also looked around and found out that there are still a few bugs in that format, but I can't tell whether or not they're game-breaking.
In Windows often stuff are more difficult, this is one of the reasons I don't use it. However, getting the hands on the original CDs does not seem an easy task. Not even via shady ways.
I was probably not clear in the first post. Yes, ScummVM does support the game, it does not support the gog format yet. One developer, known as clone2727 in the forums, is working on it. They did not say anything about when it will be done though.