I'd suggest you enable telnet to your box via the cgi script telnet hack then manually change the permissions of the files.
To telnet in, put the following two lines into a file called telnet.cgi:
#!/bin/sh
/apps/bin/utelnetd
Put this file into the web directory on the router and "view" it via the web page located on port 8080 (i.e. http://192.168.1.1:8080/telnet.cgi). This will open up a telnet port.
If you've not added additional volumes, you'll find your shares at /shares/MYVOLUME1. Just "chown -R username sharename" from there to bulk change the owner of all files.
If you've added additional volumes, they'll be mounted on /shares.
- K.C.