Funny, I had nearly the same thoughts this week! With all these cables and IO devices the N900 just doesn't cut it if what you actually need is a laptop.

Two years ago my suggestion for a distribution would have been Slax:
http://www.slax.org/

Unfortunately Tomáš Matějíček is too busy right now to maintain that distro anymore and Slax is a typical one man show.
However, there's a similar distro (I think it started as a fork but I wasn't that active in the community at that time) called Porteus:
http://porteus.org/

I just started toying with this distro so I can't give you any detailed info yet. But afair all the concepts Slax users are familiar with should still be the same. I have the impression that Porteus is not quite as vivid as Slax used to be in its best time but as I said, I just started using Porteus...
edit: I just had a quick look and it seems like the Slax forum also deals with Porteus-related questions. So that might be why Porteus doesn't seem to be that active: The community is still over at Slax. /edit

To prevent knots in my brain I'll pretend both distros are the same (in fact what I'm going to say is valid for Slax):
- It's a squashed Live CD of 200MB (Porteus has 300MB) that has a script which allows it to do a frugal installation on a USB stick, HDD, whatever...
- A frugal install allows persistent changes or an unmodified system depending on a boot option.
- Software is usually provided in "modules". These modules are compressed archives that should bring all their dependencies and can be downloaded and (un)installed at any time. I was told it's much like how software installation on Mac OS works (since I hardly know Mac OS I can't judge that).
- These modules internally basically look like debian packages but without the DEBIAN directory.
edit: Slax modules should usually be compatible with Porteus. /edit

I don't think running Easy Debian the way you suggest would work well because it's an armel architecture. So to run it on another computer would either require that to be armel as well (which limits it to a few netbooks which are hardly faster than the N900 itself) or an emulator like qemu (which will most likely cost more performance than the underlying x86 system would add).
What should work is to mount the Easy Debian home directory into an x86 Debian system on your N900's SD card.If the x86 system has the same packages installed like Easy Debian you won't be able to tell the difference.
However, that approach has two flaws:
1. The SD card is very slow. I just installed a class 10 SDHC and I'm still stuck at 16MB/s (r) and 4MB (w) for sequential 1kB blocks. There are USB sticks out there that easily double these values.
2. Debian has no frugal installation option. In most cases the system should still boot on most computers but it's not quite as universal as Slax when it comes to detecting new hardware (e.g. another WLAN adapter).

bottom line:
Let's face it, the N900 as great as it is as a *nix tablet makes a poor USB stick just as it makes a poor phone. It's bulky and slow. Just get a regular fast USB stick for your OS! I have Slax on my USB stick for 4 years now and it works fine. Don't try to force everything into one device! Some things are better separated. I know what I'm talking about. I always have a swiss knife, a lighter, a flashlight, a USB stick and now my N900 with me.