I have this exact same problem with my N900 (getting a replacement from Nokia because of a dead pixel). What i did was to just create big files that fill upp all remaining space on the devices, after I removed the passwords by normal means (rm):

If thats to slow, like when filling up the 26GB partition i did
to fill it with zeros instead or pseudo-random data.

dd will give an error when the device is full. In my case I had to run it several times on my 26GB partition considering its formated with FAT32 which only supporst 4GB files.

However there might still be a problem with this, atleast with normal SSDs, I dont know how the flash in the phones are constructed.
In SSDs there is extra space to replace weared out cells on the device, which I think is inaccessible by normal means. If a cell is weared out it might also be so that you can read from it but not write, which means your password would permanently be left on there.

Though this is only theoretical, I might even be wrong :P
I have no practical tests to back up my claims.