I'm contemplating taking the next step up and acquire an F-2b.
Looking at it there's TONS of empty space inside, which makes my modifimagination running..
I'd like to add the functions of the Series powersupply, and the guts of a GOOD stereo DI-box in the F-2B.
Has this been done before? Any opinions/advice?

The only mod I have made to the F2-B is moving the on/off switch to the front panel.  
You are right there is heaps of room in there. The only problems I could imagine would be electrical noise isolation difficulties and the amount of panel space available for the input socket.  You would have to move the on/off LED.

Yes, that came to my mind as well. But to be honest, do we really need four input jacks, two for each channel?
Noice would obviously be a concern, but there should be ways to fix that. (Shielding?)

Noise shouldn't be a problem if you are careful with your wiring. As far as the DI goes, a couple of Jensen or equivalent transformers and you're pretty much set.
 
Also, there's nothing that says the input has to be on the front.
Sounds like a fun project, keep us posted!