I just realized that if I had a coupler with the gas check valve removed and a hose attached to the gas fitting, I could use that as a blow-off tube and ferment directly in the kegs. Would that work or am I missing something?

Thank you for the detailed response! I’ve never looked that closely at couplers, but some quick browsing shows that the gas lines are attached with fittings that look to be removable, so that should provide access to the check valve. I’m cleaning carboys (and the spear-less keg) with a pump that has the carboy rest inverted over a tube with a CIP spray attachment. It sounds like I should be able to use the same setup to wash the keg without removing the spear, once I get hold of a coupler.