Crew assigned to the DC slots, and Weapons slots "are not" always on duty.
they will perform maintenence and sleep...they do not have to be moved.
If you select Battle Stations they will be activated and the box where the slots are will be highlighted much like the standing watch, however they will stay on station "activated" even after you secure from battle stations. (the DC that is)
The bottom line is you can keep them in their slots
When you are surfaced its a simple matter to crew the guns and tell them to "fire at will"
After further thought I guess I see your point...but I don't think it can work that way.
Cheers