"Hallvard Tangeraas" wrote and I slightly rearranged ...
> English schools a lot I feel (and have been told by others) that my 
> language skills are just as good as a native speaker.

Well you _write_ better than half the native speakers in this
newsgroup - probably including me. 

> Having visited Japan and liked it a lot I've given thought to the idea 
> of staying there for a year or two (and who knows what next). Speaking 
> to other gajins along the way I've understood that the easiest way to 
> get work is to apply for an English teaching position when you don't 
> speak any Japanese.

I'm not sure what else you could do without speaking _any_ Japanese.
 
> I've been trying to find out more about this, but it looks like it's all 
> geared towards *native* English speaking people in their twenties, and 
> I'm neither: I'm from Norway and in my late 30s, but.... having attended 
> I don't have anything to prove that, so I'm wondering if I should take 
> an English course at the local university, just to get the papers. Not 
> sure which course to take though...

Hmm, I don't have any direct experience but I would guess that 
certification would be a good idea.  However I think you should be able to 
take some sort of test without having to actually do a course given that 
you already can speak English.
 
> Does anyone know if it would be possible at all for me to get a job like 
> that, with my background and age?

I came across a site somewhere in Japanese aimed at Japanese 
speakers wanting to teach English ... if they can do it I'm sure you 
/should/ be able to.  

> I'm also considering an introductory course in Japanese at the 
> university, although I have no idea how hard this is. It's a two 
> semester (one year) course.

I personally would view it as a waste to go to Japanese for "a year or 
two" without attempting to learn / use Japanese.

I'm a lousy example though because I've learnt a lot of Japanese
without attempting to go to Japan. ;-)