I finally found the answer on the DD-WRT forums and it's annoyingly simple but I would never have guessed it in a million years.

The username has to be entered in UPPER CASE. So while smb://share@192.168.1.1/public didn't work, smb://SHARE@192.168.1.1/public did!

Just wish I hadn't spent a day trying to get find that out!

 Considerations

For the partition, in its entirety, at the server you grant read and write privileges to everyone:

• may be comparable to guest access, which does not require authentication.

Consider the following possibilities:

• some communication, or attempted communication, by Windows 7 may be anonymous, without authentication (whether/how such communications would be logged by the server in a non-Server build of the OS, I don't know)
• writes, or attempted writes, by Windows 7 are inappropriate for something at/around the root of the partition.

Suggestions

Increase the verbosity of logging for SMB

This may be easier to achieve with a Server build of the OS.

Diagnosis at the server when the client perceives a problem

Enable the stackshot daemon then use the key chord for sysdiagnose

Preparation: enable the daemon

sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.stackshot.plist

When the problem occurs: use the key chord for sysdiagnose.

For at least ten seconds after the chord, touch nothing.

After Finder brings to front the result of sysdiagnose: decompress the archive, consider the files that comprise the diagnosis.

References

stackshot(1) OS X Manual Page

sysdiagnose(1) OS X Manual Page