When hosting a mission, after creation of the net session, You can choose a subscribed mission on Steam (orange font in Arma3 menu). since last Beta Steam update (24/11/15), I can't download the file. I can still access to the Steam workshop, clicking on the blue button and link is correct.
This seems to be a beta Steam interface problem. But, I can't shift to the current standard Steam interface as far as there is the persistent known bug of never-ending file download in queue.
So, in these conditions:
- Arma3 tools Publisher disabled for scenarios,
- Official release of Steam bugged,
- Beta steam interface disabling download from workshop,
it seems very hard to host some workshop missions, especially, SP / MP ones.