I have converted a few harpsichord tracks into ogg (Q10), which sounds excellent and play well in foobar 2000 0.8.3
only problem is the nominal bitrate seems to be ~540kbps and sometimes reached 8xx kbps in VBR packets display

bitrate = 539
channels = 2
samplerate = 44100
bitrate_nominal = 499
codec = Vorbis
vorbis_vendor = BS; Lancer [20051121] (based on aoTuV b4b [20051117])
----------
12619656 samples @ 44100Hz
File size: 19 284 396 bytes


Is it because the frequency spectrum is extremely wide?(from 0-22000 hz without big trough)

Harpsichord tends to higher the bitrate compared to the expected one, at least with formats like Vorbis and Musepack (with other formats, the same tracks tends to do the opposite: WMA & WMAPro for example).

Vorbis is VBR. Q10 correspond to an average bitrate of 500 kbps, not the maximal one. Some frames could jump to more than that (I don't know the max size) and final bitrate could also be higher with "difficult" material (like several harpsichord tracks).
If you play with advanced options (impulse-noistune) you may get even bigger files.

Once I managed to produce an ogg vorbis file with an average bitrate of about 680 kbps (but I'm not sure what encoder was that). It was purely for fun though.

Frame size certainly isn't limited, it can go as high as the encoder thinks it needs. So theoretically the average bitrate could end up beyond 1000, though I don't know if that's possible in reality.

Well, one could create a frame-precise synthetic sample solely for that purpose…