After looking into the JeeLib code, it turns out that the rf69 compatibility code is completely broken in the version I had - and I am pretty sure that was the master version of the github repo:
https://github.com/jeelabs/jeelib

After browsing the JeeLib forums for a while it appears that the RFXConsole branch is actively maintained, and using that branch instead sending works out of the box :frowning:
https://github.com/jeelabs/jeelib/tree/RFxConsole-RSSI

Franck