...well, let's try. I hope I'm not wasting Your time?
If you feel more confident using a regular expression capable text editor, there is another approach that may work.
1) Select some images
2) File -> Export with the Text Exporter template, put {filename},{caption} in the middle text field, everything else empty/unchecked but do select "Save all as one file" and click Export.
3) Load the exported text file in a text editor that can do regular expression search and replace
Here is a fictitious sample for such export from three selected images, one line per image:
L1000398.DNG,CONT-1,CTRY-A,ST-01,CITY-1,Descr no commas.
L1000399.DNG,CONT-2,CTRY-B,ST-02,CITY-2,Descr, one comma.
L1000400.DNG,CONT-3,CTRY-C,ST-03,CITY-3,Descr, one comma, nono, three commas
The format is
<filename>,<continent>,<country>,<state>,<city>,<description>
The last five fields are from your current caption.
Now use a regular expression find and replace to modify those three lines into
L1000398.DNG Descr no commas:CONT-1,CTRY-A,ST-01,CITY-1
L1000399.DNG Descr, one comma:CONT-2,CTRY-B,ST-02,CITY-2
L1000400.DNG Descr, one comma, nono, three commas:CONT-3,CTRY-C,ST-03,CITY-3
Note that a TAB character now separates filename from the new caption string. PM wants a TAB separator.
Also note that I used a colon (rather than a comma) to indicate the end of the description in the new caption text.
I attach two screen dumps that show how I did this in the Notepad++ editor for Windows. Any text editor capable of regular expression search and replace should be able to do the same, though regexp syntax may be different. Please ask if you find the search and replace expressions intimidating. I am on the road and may not be able to answer the next few days, but I am sure Hayo and others understand it and may help.
Save the result as a new text file. Then use the new text file as explained in reply #9 by Kirk Baker in this thread about "Bulk caption edit":
http://forums.camerabits.com/index.php?topic=11149.0