ps2 bus
Although the PS/2 is bi-directional,
but only the 'Device' side could send 'CLK',
and the 'Device' has to respond (out CLK) to 'Host'
when the 'Host' issues the Request-to-Send (RTS).
The 'Host' could drop the CLK line to set it is busy,
but the 'Device' always has to standy to serve 'Host'.
Otherwise the 'Host' will think the 'Device' is dead
when RTS is on the bus (CLK=1, DAT=0) after timeout.
As your application, you could use a CPLD/FPGA to solve,
but I don't think that it is cheap and easy than uC.
And some USB-to-PS2 has very tight timing on PS/2 bus.
FYR