Using a mort as a tester in the building port is actually a god idea, since you can watch the effect on the mort with your imm, and just teleport it canywhere you want it. That's what I mainly use my own mort Emma for.
Just create another character in the buildport and then log it on at the same time as the imm.
Multiplaying is only forbidden in the Gameport, not the buildport.
You might have to ask some high level imm to advance it to level 50 for you however. Ask any imm you see in the Gameport to comer over and do it.