Their are ways to append some text to a filename (add "--large" for example),
See IM Examples, File Handling,Filename Percent Escapes
http://www.imagemagick.org/Usage/files/#save_escapes
but their are no string manipulation tools to extract a sub-string or modify strings (properities actually) within IM at this point.
Your best bet is to use some scripting solution...
IM Examples, Basics, Mogrify, Batch Processing Alternatives
http://www.imagemagick.org/Usage/basics/#mogrify_not
I have not even considered string manipulation for IMv7 properity handling, as I am having enough trouble figuring out how to add number formatting.
See notes on possible solutions in
http://www.imagemagick.org/Usage/bugs/I ... tml#format
A few of these may be expanded to sub-string and RE modification, but really it is outside IM scope.