Custom Serial Device

Is there a way to send a custom serial command (one time - not repeating) to a custom serial device when a button on a controller is pressed?  I was able to get as far as looking at custom serial devices, but from there it appears I can only send serial data on a repeating basis or when some value has changed.  I can't seem to find anywhere that I could trigger sending serial data from a controller button?  Also, I need to send Hex values, but it looks like all data is sent as Ascii...is there a way to do this?