I present the below page as an example which shows the screenshots of one gray/black market phone kiosk unlocking device which permits changing the IMEI and lists the N900(N9 too) as one phone it can hack. I believe it is a JTAG cable. A hack is in the wild but it is exclusively for those who receive stolen phones, not for us.
https://www.infinity-box.com/
Criminals already have their hack, but not individual owners of N900s.