There is an Arm Linux build of "Write" right here [1]. I'm curious if it would work on the remarkable as is.
I've never side loaded anything on the remarkable, but... I do own one.. so it's worth dusting it off and trying.
Thanks again for the link to the reHackable repo. Hacker News threads seem to cool off quick, but if I have any luck, I'll report back here.
It's linux under the cover that you can connect to if you know the password (in the settings).
There's a whole bunch of apps that you can side load into it.
https://github.com/reHackable/awesome-reMarkable
I'm not sure there's anything preventing you from installing write by stylus labs other than that no one appears to have ported it to the device.