I've thought about pretty much the same thing, but I came to the conclusion that, while it might be a fun experience to implement, it has no practical use. Why is that?

First, think about when you want to actually use the N900 as an I/O device!
It is never when you have a "proper computer" with an actual keyboard and a monitor around. Let's face it, while the N900 can do "PC I/O", with its tiny keymat and small low-resolution screen it's very bad at it. It's so bad, that any worse would be useless.
Actually, even the tiniest of laptops will be better at I/O (GPD Pocket anyone?).

So you want to use the N900 when you need a "proper computer", but you don't have one around. This will usually be mobile scenarios when you're "on the move", where even something like a GPD Pocket will be too clumsy.
A compute stick, while being small on it's own is pretty useless in this scenario, because you'll need at least an external power supply. So at least you have to plug it into a "powerbank".
You'll most likely have some sort of bag with you if you carry an N900, a compute stick and a powerbank. So why not put a GPD Pocket (or an even more conventional laptop) in it instead?

I can imagine putting the N900 in one Jeans pocket (happens quite regularly), a powerbank in the 2nd (I don't have a powerbank, so this doesn't actually happen, but I could see myself doing it) and the compute stick (or a slightly bigger and sturdier Intel Compute Card or Zbox Pico) in a 3rd pocket. Then I hook up the powerbank to the PC from one pocket to another.
Quite frankly, I don't see myself doing this, because if I have no bag with me (that could fit some laptop), it is usually because I'm doing something where a bag would bother me. In that same situation I wouldn't want to have two electronic devices in my trousers connected to each others via a cable.

bottom line:
The N900's key strength is being a (bad) "proper computer" on its own. If you have to add any element to it to accomplish that task, you lose this feature and are better off, looking for an entirely different solution.