they could at least try to test if user input a transition altitude in its FMS take off phase and approach phase, and use this one until they find a solution to get the transition altitude directly into nav data.
code like:
if(fms.tl not null) then tl=fms.tl else tl=18000.
This can let the user define itself the transition altitude to the ATC as the user can find it on charts or know its own country transition altitude. if not the sim just use the default one.