Thanks to Kenroy Harrison for pointing this out.
The TEA5761 FM chip on the N800 supports the Japanese band by setting the BLIM bit but this is not supported by Nokia's driver code (Nokia doesn't sell the N800 in Japan anyway). So I have built a patched kernel enabling setting the BLIM bit.
The new release of pyFMRadio adds support for this kernel and allows switching the FM band.
If you have a N800 and are currently in Japan, please test this Python code and report if it's actually working:
from FMRadio import FMRadio
r = FMRadio()
for freq in r.scan():