Hey Uzi,
It will depend on what you need your slave to do, and at what rates your clock will be running. If you just need the slave to send back predetermined data on the clock edges, we can do that with a device with correlated DOI(like M-series devices). M-Series can output correlated data at up to 10Mhz with an external clock. Those devices do support change detection so you could write/read on the falling and rising edges. If you need it to truly simulate a slave device and act off of commands, then you'll have to allow enough delay for the slave to read in the serial data, translate it to the appropriate word and generate the appropriate response - depending on what rate and time between your commands this can be very hard over the PCI or USB bus and up through the OS.
If you need a more deterministic solution, one of our FPGA devices would definitely be able to act as a slave device. Again, it would depend on the rates for the performance though.
Hope this helps, let us know if you have additional questions about the devices or how to implement the communication.
Cheers,
Andrew S