I'm afraid GEMUS is a bit too limited to make the 7-Zip 'on the fly' use possible
One problem is that if you use a variable in the 'Run_Program' function and it contains spaces GEMUS automatically adds quotes around them, which makes it nearly impossible to tell 7-Zip to extract the file. The only workaround is to use paths and filenames without spaces (and 'Program Files' is a good example for a path with a space...
Also deleting the extracted file after the emulation is not possible, which means that all the extracted files will stay on the harddisk or there have to be a cleanup at the start which removes old entries.
Maybe it's better to include the files seperatly then to mess with GEMUS too much (it will also make the requirements more complicated).
If you want to have a look for yourself Retroplay I can give you my (very) experimental script with some explanations.