Somewhere, I've got a wiring diagram that's intended to convert ALL the necessary KVM + Serial Port signals of an ADB Mac into an octopus cable for the DB-25 connector. I haven't tried it yet, but it may work out well. I've collected several Centronics connector A/B, A/B/C and A/B/C/D switchboxen over the years. The extra lines and better shielding of Parallel Port cables could be a huge benefit, but they're not as readily available.
Using the serial port lines on my DB-25 converter for shielding might make the signalling more reliable, dunno.
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The most common denominator nature of PS/2 would be the point of the diagram I posted above. PS/2 is a fairly efficient solution to KVM reduction of peripheral redundancy for a PC/USB Mac/ADB MAC installation.
PS/2->USB connectors are very inexpensive, while the http://Griffin Imate adapter in my setup is less so, but still available new and used for fairly reasonable prices.
That said, my Digital Audio didn't get along well with my Imate, so I wound up using the original KBD and Mouse or my favorite scrolling, two button MiniMouse along with the PS/2 set. This wasn't a problem for me. I didn't have the severe space restrictions of some here when I was using the setup posted above as my daily driver.
I'm also lucky to have purchased the OmniView Mac connection kit in the diagram above along with the three Mac A/B/C KVM switchbox back in the day for the rugger's Quadra 630/386 Clone A/V/HomeWorkStation setup. Pricing for both are insane of late.
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PS/2 KVM switchboxen should be fairly easily adapted to VGA/ADB KVM status using panel mount/soldertail S-Video connectors for ADB on the KBD connector and possibly USB conversions of switchbox cables from the Mouse connections. I've collected several mechanical PS/2 KVM switchboxen for this purpose. Electronic KVM switchboxen would be better and they are still available NIB, or were fairly recently. These are far superior to the mechanical variety.
New USB KVM switchboxen would be the ultimate in availability, so a USB->ADB adaptation would be the most useful as I see it. Macs don't like having KBDS/Mice appear and disappear on a switchbox, especially the rodents. I keep an alias of ADBReset on the Desktop of all my KVM switched Macs in order to run it from KBD commands when the mouse function drops dead. So Mouse/KBD emulation in a Mac Converter or switchbox would be very important, if possible.
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The really ugly solution I've been considering for KBD emulation on the ADB side is boxing the controller boards of the much maligned AppleDesign KBD in a KVM converter for each ADB Mac on a PS/2 KVM solution for the Mac side. I've collected several of these ADB MushBoards and reassembly of the KBDs would be no problem.
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Maybe I'll play some more in Illustrator later today, I'm not really expecting that anyone will be able to parse much of this post. :/