Hi Thorsten
I've noticed that pois are only searchable in mapsource if they appear in the mdr.img file.
The categories are directly linked to the occurence of a poi type in the mdr subfile.
This may not be what you are looking for but I'm examining the mdr structure myself and have written a small binary tool to extract the pois ( not addresses); it only reads NT and non NT mdr.img files.
http://www.pinns.co.uk/osm/bits/mdr_pois.zip
Interestingly,
a) TOPOs only seem to include pois with 1400+ range
b) There is another byte attached to each indexed poi - mkgmap seems to set it to 0 - No idea what it does but I have seen values 1-16 elsewhere.
Nick