I can now successfully mount my USB stick and here is how I did it:
Warning: only for experts!! It may damage your USB device.
This not the real hostmode. It is an unstable hack to get some debug information.
1. install the kernel from (removed. insiders know where to find it)
(contains MohammadAG patch and hcm's i2c debug patch)
2. boot and connect N900 via USB cable to PC, select PC suite mode
3. execute the script
http://maemory.com/N900/kernel/usb/usb
4. connect your stick via USB F/F adapter to N900, wait a moment

The resulting syslog excerpt is here
http://maemory.com/N900/kernel/usb/syslog.gz
for this I enabled USB debugging 5 before connecting to PC,
so that we know what the driver does when switching to peripheral mode.