In Star Control 2 part2 the full game version of Project 6014, You can hear about the existence of these guys from the Melnorme. However in Star Control 2 part 2 the demo version of Project 6014 the Melnorme are willing to give the Game player some sort of reward in credits for the information about Pre-historic space civilizations such as this one.
During the Dnyarri Slave Empire era (20,000 to 17,500 BCE) during the Earthling pre-historic eras, and by the time when the Ur-Quan finished the First Doctrinal War, the Xen-Weyi had mastered planetary ecology for the sake of reviving their own planet. They were able to create a "genesis device" that is capable of turning the surface of a barren planet into fertile soil and to give a jump start to vegetable life. The Mael-Num -- passing by as they escaped from the Ur-Quan races -- made contact with the Xen-Weyi and were particularly interested in their invention, however the Xen-Weyi were reluctant to give away the technology. Nevertheless they sold some information about themselves for bio-data.
Shortly after the passing of the Mael-Num, the Ur-Quan Kzer-Za arrived and subjugated the Xen-Weyi, who chose to become fallow slaves. It turned out that their homeworld was full of ancient artifacts older than 500 years, so the Kzer-Za wiped the life out of the whole planet, leaving it with little more than it's atmosphere and oceans. The Ur-Quan Lord offered to relocate the Xen-Weyi to a different planet, but they wished to stay. After the shield went up, the Xen-Weyi activated one of their genesis devices and turned the planet fertile, but they didn't calculate with the red glow of the shield and actually the original flora and fauna samples were unable to hold out for long. The Xen-Weyi realized that magic tricks won't work here, so they decided to improvise and use the Mael-Num bio-data to build up a new ecosystem in a long, selective process which incorporates the slave-shield as an important factor.
Present day (2160s CE)
About 20,000 years later in the 2160s CE the Xen-Weyi had established a society fully harmonized with the ecosystem they shaped, although they had a change of mindset, turning them from economic engineers into simple economists. Their system demands self-disciple, but in return it grants a comfortable life for all Xen-Weyi. They're so used to this system that going beyond the slave-shield doesn't even pass their mind, and even if it does some time, the thought is dismissed because such action would only break the comfortable balance, compromise their ecosystem and "the slave-shields are impenetrable anyway". They're fully aware that there is an outside, they just don't want any of it.
WHAT HAPPENED TO THE XEN-WEYI STARBASE?
The Ur-Quan uplifted the Baul to help fight the Ur-Quan Kohr-Ah and also look after and take care of the Xen-Weyi sort of like a Pre-Historic Ur-Quan Earth Guard, while they're fighting the anceint Alliance the Sentenent Milluiu in exile. After the Second Doctrinal Conflict, the Lurg told the Baul that the Quans are gone and they're free to plunder the Foon-Foon, so the Baul left the Xen-Weyi starbasers alone. As a consequence, all 1,500 Xen-Weyi men , women and possibily chilren all died. So everyone on the Xen-Weyi StarBase died by circa 16,000 BCE. A reminder about what would have happened to Hayes and the Earth Starbase, would the Captain not deliver radioactives at the last moment. After the Xen-Weyi starbase died the Baul just like the Spathi and Ilwrath abandoned the Earth, eventually the Baul abandoned the Xen-Weyi homeworld.
The Xen-Weyi still got something on them that the Captain needs: the Genesis Device. So the Captain has to crack the Xen-Weyi shield anyway, not knowing that destroying the shield puts the ecosystem out of its balance. The Xen-Weyi doesn't appreciate this too much and demands the Captain to put back their shield, a perfect time to blackmail them for their Genesis Device! Now the process of putting up a slave-shield is a bit costly, so the Captain might choose not to do that, which means the Xen-Weyi planet becomes wasted in about a year of ecological regress if it's left without a shield. In that case, the ruins can be explored for the Genesis Device later.
You might ask: so what? It's a win-win situation, you get what you want anyway. Almost true, but there are moral differences, you get delayed by a year and there's something else on top of that. The the Lurg know the secret location where the old Xen-Weyi ships are stashed and they're just waiting for an opportunity to give them back to a very angry Xen-Weyi race who -- losing an almost utopian existence, and forced into a strange situation they couldn't control -- want revenge against the joker who just wasted their life. In other words: if you want one less enemy at the end battle, you stack up enough resources to put back a nice red shield that would glow over their heads for a million years, and still you cannot recruit them to the NAFS.
NOTES FOR WRITING
The Xen-Weyi are close-minded, pompous and condescending. They have a radical economical conscience and a highly negative attitude towards the exploring, exploiting and expanding nature of space-faring cultures. So everything the Captain DOES and more. They're also angry at the Captain for turning off the Slave-Shield.
NOTES FOR VOICE ACTING
The Xen-Weyi has a natural feature that makes them talk like they have snuffles. Shut your nose while talking to the mic.