No offense, but I can't see any situation where it would be a good idea to blow air from a regulator into a bag containing an open camera housing. The possibility of blowing sea water into that open housing is going to outweigh any benefit of getting dry air into the housing. Even a "dry" regulator can have pockets of water in its nooks and crannies. Maybe if the dive boat / shop has one of those blower attachments but, even then, compressed air can do other damage, like pushing an o-ring out of its groove or pushing a sand grain down behind an o-ring. David's toilet paper or my silica gel sachets is going to be a much safer more reliable solution.
I actually have used a cloth covered flex tip valve on a power inflator hose to carefully do this on several occasions out of need (had to risk opening the housing for some reason on a open boat.) Never used a 2nd state regulator directly to do this personally since I had the tip. This was on a large volume housing where no typical silica gel pack is going to help ( not a go pro.) Obviously I didn't blast full-flow 100psi air in unabated, and expect most handling underwater cameras would think about why that isn't a good idea. Maybe I should add a disclaimer here. lol
There is no beating letting A/C'd air fill the housing before you close it up if you know the dew point of that air is safely below the coldest temp you'll experience during the dive. It doesn't matter if you take it outside after you close it and it sweats like a ice cold bottle of beer would, as that is on the outside (though it can freak some people out if they don't understand that.) Even as the air temp inside the housing rises or falls with the ambient environment, the dew point remains constant inside the housing as long as it's not opened.