Plastic under secondary board can be easily removed completely - and indeed, once I've fixed secondary board via two-side adhesive. worked great, and created much space for relatively "big" (in diameter) cables.

Of course, if Your way works for You, it's absolutely OK - I've posted above, just as tip for others, who may want to follow "full-sized-port way, without deforming N900's case too much (or, particularly, want to do cable's method. It seems to me, that if someone is determined enough, it's possible to avoid cutting any plastic at all (other than removing plastic thing'ie from secondary board, as mentioned earlier).

/Estel