When getting a raw chromoly frame and fork powdercoated, what needs to be done beforehand? Mask off the bottom bracket and headset areas? Anything else? It's a threadless fork. Should I mask every part of the steerer tube, from the fork crown race on up? What do you mask any of the areas with?

the local powdercoater doesn't want you to mask anything.

Anything with threads needs to be masked, PC is tough. Does it have any bosses, like brake bosses or shifter bosses?

Generally, I don't think I would get a bike PC'd by anyone that hasn't done it before. If they have done it before, they know how to mask.

The powder coater will handle any masking with a method that will withstand the heat of curing the powder. My powder coater has an assortment of silicone plugs for bottle bosses, heat resistant tape for areas that need to be taped off and I've seen him use a vacuum with a needle sized nozzle to vacuum away the powder from areas that don't get powder.

+1 Use a powder coater who has done bikes before. they will media blast the frameset and they use special masking tape that withstands the media blasting. If they don't know how thin walled the tubes are, they may be too aggressive and destroy the frame. A coater who knows bikes will likely have 5mm bolts on hand to plug the water bottle or rack bosses and other threaded bits and they'll know to mask the BB/HT. Let them know of other sensitive bits like brazed on headtube badge or such. The heat may discolor some of these items but they can be re-polished.