I know med supports polygon groups, but I do not think it supports multiple models.
So your room and furnature will be one big model in med, and each polygon group
will not have a center other than the origin.

You could put a vertex at the origin of each piece of furnature before
combining the room and exporting to med, then you could move your mouse over
this vertex and note the coordinates in meds lower left to find what the models
offset would be.

It might be easier to create a level editor to create and place objects, then
output their model type, pos and rotation to a text file. I did it that way
once for a huge terrain with trees and rocks.