What about using colored labels as workflow indicators. For instance, assign a color to all those original images you've made Lightroom edits to. You can still append the file name for those TIFF, PSD, or JPG images you've created as derivatives from the originals.
Personally, since all my camera originals are raw (dng actually), anything else produced (tiff, psd, jpg) are derivative so I don't necessarily need to append the file name. However, I will do that when I create specific print sizes. Then I'll add something like "_12x18" to the end of the file name.
Another option is using keywords, as you've described. But it seems this might be a pain. And what if you decide to hit the "reset" on an original? It's much easier to add or remove a colored label than a keyword.
You could also use custom metadata fields. John Beardsworth's Search Replace Transfer plug-in
adds something like 18 custom fields. I've found that very useful (the whole plug-in is remarkably useful)