@Gorgonops This would be an easy option for the BeagleBone Black, which has a lot more of the PRU lines connected to its expansion headers. For the PocketBeagle, you could definitely swing it, but it'd be tighter. (You get 14 lines that can run in either direction, six that can be inputs only, and two that can be outputs only---presumably we would allocate some of those six input-only lines to getting input from the data lines, though several other schemes are possible. These lines may not all go to the same PRUs, though---in fact, they don't---so that could be an important hassle.)
Other than the line-to-PRU riddle, I guess the main disadvantage at that point is the complexity of the adapter cape; the part count and of course routing it all---more lines mean another level translator of some kind (though for inputs only a voltage divider might be OK). I also wanted the cape to be pretty general, and tying the bus input and output lines together moves toward specialising the cape to hard drive emulation. As it is, the cape commits pretty strongly the other direction---it includes this pattern of pads for surface-mount jumpers that will hopefully allow people to customise it to their needs in certain ways (installing pull-ups, termination, etc.).
(All that said, I am considering whether I could ditch the pads and make a task-specific version of the cape that's only two layers instead of four, which would be a significant cost savings for the PCB.)
The solution you describe with the 74LSwhatever, meanwhile, is exactly what happens inside the real ProFile
Hrm. Just ignorantly spitballing, could you get around this problem by using separate input and output pins, IE, define two separate 8-bit groups on the PRU (one for input, the other for output) and alternately use them to drive or listen on the common bus lines based on the state of the PR/-W signal? (Note I'm saying this with no idea if there are enough PRU pins to do the job.) Offhand I don't really see an issue with doing this as long as you don't drive the output line at the same time the host is. (If you were really worried I suppose you could put an 74LS...244, 245, whichever, in front of the output with its OE enable line tied to the PR/-W appropriately.)