Well, at the time I wrote that, I had looked for pbxnsip binaries but they were not there yet. They are now. No dead end any more .
I installed 4025 and it works fine. Thanks. Now let's see how it behaves at the next incident.
BTW. I read that the fix is to declare a 408 error on the trunks when the registering stops. I then can receive an e-mail, go into the web interface and click "REGISTER". That's much better than how it was. However, I would prefer to be able to automatically recover from an incident . Right now I am thinking of doing so by automatically "clicking" on the REGISTER link of each trunk. So I am still interested in the incantation to do that with proper authentication : 'http://<pbx ip address>/dom_trunks.htm?trunkreg=<trunk index>' and then some extra stuff. Or something similar.