Flexible dimensions/parameters in Repeat Region

Header pretty much speaks for itself. I have flexible dimensions in some parts. I assemble several of these parts and use surface references to define these flexible dimensions, then I'd like to have a BOM of all these parts that lists their flexible dimensions. Driving length/width/height parameters from relations doesn't update them to the flexible dimensions (only returns the dX values that are in the original model), so it doesn't seem like I can use asm.mbr.<user_defined> to show these dimensions. Seems simple enough, but I just can't seem to find it. Any thoughts?