Thinking about Gemp's order of operations when it comes to playing cards makes me think it would operate properly. When you play the card from your hand, eligible dwarven companions will have their cards outlined in blue indicating they can be chosen for spotting purposes. Given that part of the spotting is checking their damage + value at this point in time, I would guess that it correctly determines the X value for
BRC. So, after a player clicks a dwarf for the spotting requirement, that dwarf will be given two exertions and the opponent will then have to choose X shadow cards to discard where X is the value determined during spotting.
However, it has been a while since I have played on Gemp and certainly longer still since I have played a dwarven freeps on Gemp. Thus, my suppositions about the way Gemp actually performs may be flawed. Sorry I can't be of more help here.