Although automatic BB conversion would be nice, perhaps the best solution is to simply not use the Steam description in-game:
if you've subscribed via Steam, chances are you've had the opportunity to read the full Steam description - thus in-game may only need a brief rehash, or the otherway around; either way, it would give the mission designer the final choice in representation.
In addition to that point, urls i put in Steam are not likely to be viewable in arma anyway (or not desired), no sense in cluttering up the in-game desc. with non-functioning links.
Thanks @muf, looking forward to your solution!