some fascinating insights here, thank you!
had not occurred to me that the rolls could be edited, but of course, it's possible they could punch extra holes after the performance. perhaps it was even possible to play it back and "overdub".
i tried playing lhat left hand pickup Eb-F-Ab (all octaves) but I found it impossible to play accurately, even at slower tempos, and was baffled how Joplin could do it, and some of the other bass patterns.
i saw a video about player pianos and apparanetly there are dynamics, recorded on a separate "channel", but this must be for all the notes being struck at the same time, not velocity per note. it must have controlled the opening of some valve to control the air pressure. the tech on these old player pianos is incredible.
i'd love to hear any other "played by scott joplin" performances if you have them.