How to Safely Delete an in-progress Time Machine Backup on Snow Leopard
After trying various techniques the only solution I've found is to connect the Time Machine volume to a machine with a newer version of OS X and run tmutil from the command line, I believe any version from Lion onwards should have this. The command to run is as follows:

tmutil delete '/Volumes/&lt;time_machine_volume&gt;/Backups.backupdb/&lt;computer_name&gt;/&lt;backup&gt;'


Replacing the parts in angle brackets with the correct values or, if you have the backup visible in the Finder, simply type the tmutil delete part in the Terminal then drag the folder onto the Terminal window to enter the path automatically.

Once this has completed, you can simply eject the volume and reconnect it to the Snow Leopard machine, which should just pick up where it left off (though it may re-index the drive during the next backup). This is okay to do so long as you don't do anything else with the drive that could introduce any incompatibilities with Snow Leopard (such as backing up with the Lion onwards machine onto that drive).

I can't help but think that there should be a way to do this by taking advantage of in inodes to locate the parts of the backup that actually need to be removed (i.e - any folder or file where the inode number doesn't match the corresponding entry, if it exists, in the previous backup). I've used this before to quickly print a list of changes between two Time Machine backups, but there's still no way that I know of to delete the other (hard linked) directories.