The EVAP system on the LS is actually pretty similar to the factory Jeep emissions setup. The fuel tank is vented into the charcoal canister which is connected to the purge valve on the intake manifold and a fresh air intake vent, both of which the computer operates. I connected the LS purge valve on my LS6 manifold to the factory Jeep EVAP purge hard-line that runs along the fuel-rail back to the canister.
For the system to operate, GM computer also needs fuel tank level and pressure sensor, which is located on factory GM fuel pump module. I decided to use a factory GM fuel module in the XJ gas tank but in your build maybe you can just add a GM fuel sending and pressure sensor to the factory Jeep can? On the GM unit, the pressure sensor just pops into a hole that has a little rubber sleeve in it.
Hth,
CJ