Pocket holes for bed frame support
I think the pocket hole screws will hold fine for assembling the headboard and foot board, but for where the rails connect to the legs of the headboard and foot board you'll want to go a different route. 

 

Bed frames get a lot of forces applied to them from different angles. Supporting weight is only part of the equation. The joints between the rails and the foot board and headboard have to withstand racking from side to side and backwards and forwards. A lot of those forces would be directed against the bit of wood at the base of the pocket holes that attach the rails to the "legs" of the bed. The problem with pocket screws is they can pull out or loosen significantly after a few years of use. Maybe they won't...but they might, and there's only so much re-tightening you can do.

The biggest problem that I see with using pocket screws where the rails connect to the legs, is the difficulty in disassembling the bed frame for moving it and so forth--especially since some of the pocket holes in the rails end up being covered up by other pieces of wood that the plans call for gluing in place.

A queen size bed frame made of solid lumber is going to be large and quite heavy when completed, and chances are you're going to want to move it someday--To the other side of the room, to a different room, to a different floor, or a different residence. (Have fun with the stairs or loading it into a vehicle!)

I would strongly recommend that you make use of a bracket of some sort or bed bolts to fasten the rails to the head and foot boards so that you can take it all apart relatively easily. 

I've specifically used the Rockler brackets in the example and can attest to their strength, etc. Those guys are very solid, and have a "safety screw" to ensure that things stay locked in place. There are many types/styles made by numerous manufacturers. 

Bed bolts are another good option, and have been used for maybe hundreds of years. (The guy in the video makes installing the bed bolts a lot harder than it has to be...but he does a good job.) You'll probably want to cover the exposed bed bolts (if you go that route) but they make bed bolt covers or you could make something out of wood to match the "rustic" style of the bed frame, too.

The brackets and bolts are potentially stronger than pocket screws and will take a bit more "abuse" over the years, too. The pocket screws might not fare as well after a few years of "rigorous movement" on the bed.