Thank you! Yes, Gravity Forms wiped their hands of it and offered no help with regards to stopping uploads, writing warnings, or renaming files before upload. My host offered to whitelist the mod_security rule, but I decided to leave it as is and include in the form a warning not to include those characters in a file for upload.
All that is to say that, yes, if you can build this error message and stop the upload of a file with a quote that will be not a bad compromise at all. I appreciate you taking the time to look into it and find a solution.