I wound up making it an option. The default behavior is the existing behavior: the user can type any filename, including absolute or relative paths.
With -onlyfiledir, RemGlk discards slashes and backslashes from the prompted filename. So the file will always be saved in the working directory. (Or a specified -filedir DIR.)
This is for the Discord bot case, where the user is on Discord but the interpreter is running on my server. No fair telling the interpreter to save a file in /home/zarf/.ssh!