I believe that tool merely acts as a DNS proxy, spoofs the PS3 servers so they point locally and perhaps also acts as an http server to send out a false update file to make the PS3 think no new firmware is available. Both of these things should be relatively easy to do on the N900, half the software you need is already installed on EVERYONES N900 as standard.

The N900 uses dnsmasq for DNS caching/proxying, you should be able to tweak that to spoof the DNS record of the PS3 servers. In fact that is exactly what I first started using dnsmasq to do on my PC at home, to do my own advert blocking for the whole LAN, I even used to have it send out fake adverts to MSN Messenger for my amusement.

You need to change how dnsmasq is started on your N900 so instead of only working locally (its designed to only respond to the N900 itself asking for DNS information, for security) and tell it to spoof the PS3 servers. Then you would simply set your PS3s DNS server as your N900s WiFi IP address and make sure your N900 is connected to your WiFi.

Likewise as you can install an http server on the N900 there is no reason you could not also spoof the update servers. However I do not know the specifics about the configuration of the N900 to assist further, but it shouldn't be very hard to do IMO.