If I wanted to simulate keypresses over a phone line, could I just play dial tones?
DTMF touchtones follow this table: -



In other words there are two tones produced hence, any DTMF/touchtone decoder is looking for two simultaneous tones. It is unlikely that you will trigger any false detection if any relevant tone produced by your dial-tone doesn't coincide.

If you look at the data sheet of the MT8870 touchtone decoder you will get some idea where you can pitch your dialtone: -



Any tone under 500 Hz will not stand a chance of being detected correctly even if two tones were generated with one being spot on a detectable frequency.

Of course there's no guarantee that an MT8870 is being used but it gives you some confidence in choosing a single tone below 500 Hz.