You can clone your OS to an external HD, boot from the external and use DU from there to repair the internal drive.
If by USB you mean a thumb drive, it's possible, but you need to install SL on the drive and then boot from it. Which would require a working DVD drive anyway!
If you can install on a thumb drive from another Mac (using the retail disc), then update it to 10.6.8 that may do the job. Otherwise go with the external HD.