The SC670 is just a renamed SC740 (which is supported) with USB. So I used textedit to find/replace the text "SC740" with "SC670" in every file inside the SC740.plugin, located in /Library/Printers/EPSON/. There were several bundles inside the plugin bundle that also needed the same treatment. I made sure to change the names of the files also.
Now my printer is recognized and prints perfectly. I'm sure there is an easier way to make the same changes using the command line, but I'm not that comfortable with many of the commands yet. Why Epson couldn't do the same thing, and add support for this printer, remains a mystery to me...
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UPDATE: I received the following from the author of this tip:
"...unfortunately, it seems that it doesn't always work, and the variable may be the filesystem format. I had originally formatted my OS X partition as UFS, and when I hacked the printer driver it worked just fine.So as with most things experimental, use at your own risk!
Last night I decided to reformat the partition and reinstall OS X. This time I used HFS+. When I copied my hacked driver back to the Epson folder, it didn't work. I figured it may have been damaged in the backup/restore, so I went ahead and re-hacked the file. That didn't work either. I don't know what is going wrong. It may be because HFS+ is saving some sort of metadata with the files when I save the changes. The size of the bundle increases significantly after I make the changes.
Anyway, I figured I should let you know so that you can either update the hint with a caveat, or remove it to prevent anyone from screwing up their printer or something."

