Good to know that this would [probably] work. I'd be surprised if nobody's tried this before, as everything seems to have been tried at one time or another (and everytime I come up with a good idea, it almost certainly turns out to have been done). Once I get my tannic / gallic acid extracted (another project), perhaps I'll try this. The only iron compound I have is yellow ferric ammonium citrate - is that light sensitive enough?

was abandoned for the mixed sensitizer, printing-out processes for cyanotype, chrysotype, and kallitype before there were photographic negatives to print, and before anyone tried Herschel's original (much faster) method in camera.
Is this to say that the two-step method is faster than the print out method, perhaps even in-camera fast? Or am I reading incorrectly?