Here are all the ones I have on ROM. I think the only one missing here is Xenophobe, which is being actively sold at Video61 right now (with part of the proceeds still going to Fandal, who rescued the proto for the community off of eBay; it had never been seen in public before that).
There are two versions of Mean 18. The one that's labelled "Mean 18 +" is a hack by Fandal in order to improve the scenery drawing routines to make the game a little more playable. The other version is the original proto, which is quite sluggish.
Bruce Lee is claimed to be an XE Proto by Best Electronics, who is still sells them.
Best source for proto information is on Tempest's AtariProtos.com.
A video of the Xenophobe cart can be seen on YouTube: Xenophobe Proto
Bruce Lee info can be found on Best's site: Best Electronics - XE Games (just search for "Bruce Lee" on the page)
You can also get a little bit of information on Atarimania.com and here: Atari Explorer - XE Video Games (page loads slow, so be a little patient -- from WayBack Machine)
XEGS Protos.zip 277.32KB