This isn't exactly the desired solution for indicating filled IPTC fields, but I use it and find it to be quite useful:
Under Edit, go to Set Info Text.
Add the {caption} variable, or the variable for whatever field is important to you. You can also remove a bunch of the variables that you may never use, like exposure compensation, etc...
Under View, select the Show Info Tooltips option (on the Mac it's Apple-Shift-T)
Now, whenever the cursor passes over thumbnails in a contact sheet, it will show a bunch of info, including the caption. Initially I found the tooltip feature a bit distracting, but now rely on it a lot since it saves me time not having to preview images just to see shooting info, or having to access the IPTC info to read the caption.
That said, I agree it would be a useful option to have some sort of visual indicator to show which images have been captioned and which have been successfully sent by ftp.
For me the desired field of reference for an indicator would be the caption field. A problem could arise if you apply a generic caption to all images upon ingest... then I suspect the desire will be for the indicator to show the ones that have been given full, detailed captions. How would one implement that?