ok, let's call this solution "USB/IP hub" (tm).
after some googling it turned out to more feasible than I initially thought:

USB/IP is implemented in Linux 2.6.28 (staging) and consists
of a pseudo USB driver on the server (the hub, where the USB device is connected)
which redirects all IO through a user-space daemon to the client (N900), which
has a simple virtual HCI and standard USB device drivers in the kernel.

The simplest solution would be have Linux (e.g. 2.6.28) on both sides.
We could already try that out by recompiling the N900 kernel and connecting it to a USB/IP enabled Linux desktop via USBnet/wifi/bluetooth.

If that works reasonably well the next step would be to look for a small embedded Linux (2.6) board which has more than one USB host port and can be powered by battery.
for example, http://www.tigal.com/product.asp?pid=1333&lang=EN (too large),
http://eshop.acmesystems.it/ or, my current favorite:
http://www.ic-board.de/product_info....0-OEMplus.html
features:
Linux 2.6.28
3x USB2.0 High Speed Host (one reserved for connecting to the N900)
10/100 MBit Ethernet (LAN port for the N900)
small size: 40mm x 35mm x 6mm, weight 50g ! (w/o USB ports and battery)
if you want you could even add a SD card reader, serial ports, LCD etc.
you could also connect a Bluetooth module via SPI and have 3 free USB host ports.

the only drawback: the price
but if you build it yourself (eagle files are free) I'm sure it's much less and
sparkfun.com is willing to do mass production for cool projects (UBW32).

It contains a very attractive USB controller http://www.stericsson.com/product/222228.jsp
and with an appropriate battery we could not only power the USB devices
but also charge the N900.

The advantages would be :
* up to 3 USB host ports with power supply
* LAN port
* no need to hack the N900 USB controller
* minimal software changes: compile existing USB/IP
code into client and server
* could also be used as a USB extender (USB over Internet)
or with other computer or Windows

cheers, (dreams to be continued in my bed)