App that does not support Smart Folders / Tags - how to access Tags/Smart Folders via Terminal or elsewhere?
What you could do is running a script which uses mdfind to symlink all files with a specific tag into a folder, and then access the files via this folder.

cd TAG_FOLDER
mdfind -0 kMDItemUserTags="Red" | xargs -0 -n 1 -J % ln -s % .


(Replace "Red" with the name of the tag you are looking for)

If several tagged files have the same name only the first one will get linked (you'll get a warning message for the others). And, of course, the commands needs to be rerun every time you add/remove the tag from a file.

PS: There is a small caveat here. Not all applications handle symlinked files the same way. Test first whether an application writes a changed file back to the original place or just replaces the symlink with the updated version.

PPS: 


mdfind -0 finds all files/folders matching the query (in this case the tag). The -0 ensures that the string passed to the next command is terminated by an ASCII NUL character (to ensure that file names containing spaces etc get handled correctly)
xargs -0 -n 1 -J % reads one line of input to build and execute a command. -J % sets the placeholder for the input line to %
ln -s % . symlinks the file/folder in % into the current directory (.)