I finally got a foam kit from Camera Solution, and changed the foams tonight.
I took the opportunity to clean the mirror, screen and prism with "absolute alcohol" (not sure of the translation, it's 100% alcohol). I was a bit nervous about the screen, but I have another one and this one was too dirty for my taste.
The viewfinder is now bright and clear, and there isn't any goo left... I'm quite happy of the way I spent my evening (I also repaired a right for the wife, which is always good for atmosphere ;-) )