Hi there,
I was not able to do automatic provisioning of Linux, but was able to access Linux machines (in an unsupported way) through RAS.
Just install xrdp to a Linux machine, and then add the machine to Parallels RAS as a "Remote PC". Then, publish it in the same way you do with a Windows Remote PC.
You will then be able to access it using Parallels Client, using Direct Access or Gateway.
(I was also able to do a rudimentary Linux "Parallels RAS agent" that reports a fake version to the console; but this is totally optional)
Even though you use "Remote PC", it will work as a terminal server, and allows multiple users to connect.
Performance wise, it will be the same as a vanilla Linux XRDP setup.
Regards