Ok, friends, finally some progress!
If I run the game in TCL with sudo then the gamepad WORKS!! No need for libmtp or supplemental udev rules.
But of course this is not a solution: It just suggests what the underlying problem might be.
I checked in Devuan and I am not part of an "input" group:
Devuan$ id
uid=1000(bruno) gid=1000(bruno) groups=1000(bruno),24(cdrom),25(floppy),29(audio),30(dip),44(video),46(plugdev),102(netdev),108(bluetooth)
I tried this in TCL...
TCL$ sudo chown bruno:staff /dev/input
TCL$ sudo chown bruno:staff /dev/input/js0
...but found that running the game with sudo is still needed for gamepad to work.
Any suggestions what needs to be done in order for the gamepad to be usable in TCL without running the game with sudo?
EDIT: In TCL I also tried loading polkit, starting dbus, then starting polkitd. Alas, no change--sudo still required for the game to respond to gamepad input.