You can use Code Replacements for this.
You'll need to create a UTF-8 text file, in tab-separated format, that contains both your file names and the caption text like so:
{filename}[tab]{caption}
Image001.JPG[tab]New York [new caption from text file]
Image002.JPG[tab]London [new caption from text file]
Image003.JPG[tab]Paris [new caption from text file]
(where [tab] is the tab character)
In Photo Mechanic, add your text file via the Code Replacement dialog (Edit > Settings > Set Code Replacements...) Then select your images in the Contact Sheet and open up the IPTC Stationery Pad (Image > IPTC Stationery Pad...) and uncheck all fields.
In the IPTC Stationery pad, locate the Caption field and enter the {filename} variable contained within the Code Replacement delimiter like so:
\{filename}\
Then apply the Stationery Pad to your images.
I recommend that you run the first pass on a copy of your images.
More information on Code Replacements can be found here:
http://docs.camerabits.com/documentation/learn-basic-code-replacement-in-photo-mechanic/http://wiki.camerabits.com/en/index.php?title=User_Manual_Flat_View#Code_ReplacementHTH,
Jerry