Wow, interesting thought, but pretty sure there's no way it would work.
Don't know about the pin-for-pin compatibility (data, power, ground), but though they are both part of the mini-DIN family and the pin layout looks more or less the same, there is a key in the ADB plug that would prevent insertion into the serial adapter. (Note that ADB cables are basically identical to S-Video cables.)
The real problem is that there is a whole lot that needs to go on to communicate to an ADB device that is normally done by the host computer. The iMate takes care of that communication, while a normal serial adapter wouldn't know where to start.
Same goes with trying to hack a PS/2 to USB adapter to work with ADB. They just don't speak the same language. ADB was invented by the Woz to be a better solution for its intended purpose and is a very different beast to standard PC serial protocols and PS/2.
For starters, ADB is a bus architecture and all communication on that bus is instigated by the host computer (which does periodic polling of the devices detected at startup for status updates), while with PS/2 either the keyboard or the computer can instigate communication by manipulating the clock line, IIRC. ADB doesn't even HAVE a clock line.
So the iMate has to mimic a host computer as well as translate the ADB protocols to USB in both directions. Bit more complicated than a Blue Cube or a serial Adapter.
