[EDIT: When editing this 'followup' part I seem to have removed my original posting. Which essentially said 'but unless I misunderstand what you're saying, my GPS receiver _does_ send that data, it can't be disabled. Because the same receiver works fine with TomTom and Maemo Mapper on the old N800, where the data is displayed.]

Followup: In addition to the GPS working fine with TomTom, and Maemo Mapper on the N800, I just installled 'gpsjinni' on my N800, and it shows the satellite data just fine. Sow the raw NMEA data looks fine (that's what gpsjinni uses).

So it still looks like the problem is the GPS subsystem in the N900 (gpsd?). It does something funny (like filtering away that data).
[Edit3: D*mn! It works occasionally! I won't go into all the testing I did, with various applications, but it definitely seems to be something very fishy going on with whatever software included in the GPS subsystem of the N900. My receiver is utterly reliable by itself and has worked with everything I've used it for, before this.]

The second problem is of course why Sygic decides there's no signal when that data is missing. The rest (what it actually uses) is there, and works with Maemo Mapper.