Two brief ideas since I'm better to give ideas than doing PR by myself
Since neo900 is a expensive niche device, and not "yet another android device", we must target the right audiance :
- people concerned by privacy/security
- open source enthusiasts

So the ideas :
- Already proposed and don't want seem to be insistent but getting a contact with security OS developpers.
It may be OpenBSD (I proposed it because I use it daily and it already has a limited arm support) or BitRig (OpenBSD fork oriented toward armv7 support - don't know how active they are) or any other security-focused operating system. Sending prototypes to enthusiast teams would help them to make their OS work on neo900 and could raise interest from their users.
Aahh, Wish I had put more than 750€ in this project... I would proudly have offered my prototype to any serious OS developper. :x

- Contacting FSF and try to get the RYF certification.
Obtaining this certification could make us benefit from FSF promotion because it would be the first "RYF phone". Since getting this certification requires to meet some criteria (see "100% Free Software", "Other Misleading Endorsements", "Cooperation with FSF and GNU Public Relations") it may not be so simple to achieve.