You can mount an archive directly as a hard drive in WinUAE.
Under "CD & Hard drives", click "Add Directory or Archive". Fill in desired device name and volume label, then click "Select Archive or Plain File". Make sure "Bootable" is checked.
You can now boot directly into the archive and simply run the demo by typing its filename in the archive (use some archive utility, such as 7-zip, to see what the name is).