I don't know if this is even possible but can I find out the disk usage of a partition with a unknown or damaged filesystem?
Wouldn't it be possible to extract this quite simple using dumpe2fs? It seems you can get some usefull numbers from the output of $ dumpe2fs <partition> |head -36E.g. "Block count" - "Free blocks" might give you a hint, someone who actually knows ext2/3 (or google for a description of it) can probably work out the exact numbers.
Just a thought, I may very well be wrong. In any case it would be interesting to know.
Sincerely yours, Emil