on 6128 according to a dissassembly of the os you can poke b1e4.
I think the value is 1 for play, 2 for record.
arnoldemu has control over the keyboard and it simulates keypresses. From the emulated CPC perspective it is like the user is typing the keys. (it does a key down, wait a frame, key up, wait a frame). The wait is for the firmware to see that the key has been released.