I think the simplest method with this kind of low speed stuff is just going to be to buy a dev board for a micro or a cheap CPLD/FPGA with the interfaces you want to be able to test and enough firmware available to make opening those connections and sending data easy. Then if you want wide ranges or AC transmission, you need a second supply, a transistor, and a cap for AC coupling, would be very simple to add on a breadboard and until you get into the few MHz range, having it on a breadboard shouldn't degrade the signal integrity significantly, especially at larger voltages.
There are certainly dedicated clock synthesizers or data pattern generators that can do this, but generally they'll be much higher bandwidth and much higher price to go along with it. Probably great candidates for what you're trying to do, but many times more expensive than what will get the job done.