You can use any of the timers to generate an interrupt at a fixed rate. The interrupt routine would then clock in/out one bit at a time. I can't fully understand why you would need to limit it to 19200 baud max? Is this so you can input/output data via the uart also? If this is the case, one would normally buffer the data - that is collect enough data to send to the spi device in memory then output this to the spi device at the speed of the spi port (be it hardware or software) and vice-versa with the received data.

A bit more information here would make things a little clearer - tells us what you're trying to acheive as a end result and we can work backwards towards a solution.