To close this issue:
Yes at least it is possible to run the X5L (remove 2nd battery pack glued to original HD) with a 32GB ATA 5 50pin SSD from Samsung with the original firmware from COWON. Didn't test Rockbox but I guess it will work seamlessly as well.
The System will almost fly, due to startup and seek times of the SSD being at 0.2 ms the benefits are really close to incredible. Ultra fast turn on and title skip in comparision to a standard disk.
But of course, - this modding is very expensive...
Ok, I will quit Rockbox as I dislike forums that will allow users to rip my email adresss from it for spamming.
Best wishes for all (besides the f* spammers).
AdT