Thank you for that information!
No need to go after the .img files then.

Yes, I managed to install Maemo Leste as described in the wiki.
The system seems to be up and running OK - I ssh'd into it and tried some things - but the UI is completely unusable?
The same with Devuan.
I also managed to boot a 2013 version of Sailfish OS. Not perfect either, and why bother for a system that is almost as outdated as Hramattan itself?

What else can I try?

Currently I'm thinking that the original Harmattan works well enough, and could be made safer by

• removing apps
• updating a few core apps, like openssh for example, to current versions

??
Maybe even enable some mainline Debian/Ubuntu repos?
But I don't see anything for the phone's architecture - which says it's "armv7l" - here? Is it compatible with armel, armhf, arm64?

I am hooked.
Never before have I had a truly hackable Linux phone.
I bought another N9 (a SIM adapter got stuck in the first one's SIM card slot ), and also an N900 while I was at it...