Just::Thread is not available for ARMEL at the moment. Porting to ARMEL is more than just a recompile, since the assembly code must be changed. It is currently on the "future plans" list, but with no due date.
If you upgrade to a site licence then I will move this port to the top of the list, and start work on it. If you need it really urgently, you can pay for a custom port.
