I'll look into a little centrifugal blower fan, thanks for the suggestion! I'm playing with the idea of just running a hose far from the worksite with no forced air. Not sure it will work since it would exhale through the same hose instead of the exit vent, but it's easy to test.
I'd expect a length of hose of practical diameter to attach to a mask to be surprisingly fatiguing to breathe though.
Don't be particularly afraid of the blower thing, I just wanted to make sure that "more isn't always better" was clearly communicated and there is most definitely an upper bound. I.E. don't go hooking up a shop vac with a blower configuration up to a half mask. A typical CPAP prescription may deliver ~10cm water column of pressure (
https://www.apneamed.org/blogs/info/how-when-to-have-cpap-pressure-settings-adjusted ) my inclination would be to set up the blower with the length of hose you want to run, to a blanked off half mask, so that there was 1-2 or so cmH2O of pressure with it blowing though the exhale valve. There will be more pressure as you exhale, but nothing crazy, and it may drop negative as you inhale, but if the blower is covering a majority of the hose resistance, nothing too difficult.
Adjusting the fan speed up or down to what feels comfortable shouldn't be a problem, just trying to get you in the ballpark to not grossly over-spec a blower, and have a safe starting point.