The issue isn't people queuing for roles they aren't specced for, it is them queuing for roles they can't handle which is generally a gear issue more than a spec issue. Probably any of the classes with a tanking form will be able to tank HM FPs in their dps specs as long as they have Columi tanking gear and swap stances.
Please don't say that, you'll encourage even more people to do it
Seriously, Shadows and Assassins especially should not try this unless they are confident in the abilities and gear of the rest of the group. Without Kinetic Ward / Dark Ward, and the damage reduction they get from tanking spec, they will make things significantly more difficult for their healer.