It's truly missing.
I'd check out the various plug ins to see if any supply this missing capability.
A workaround that does not require a plug in would be to use insert file in your script for the user to use to find and select a file already in the "save" location. If you specify the "store a reference" option, this enters a filepath to the file into the container field and you can then extract the filepath to the folder from there. Not the most user friendly option,but it works.
Another option is not to allow the user to pick their own location but to set up a standard folder on the desktop or in documents to which all such files will be saved.
Thanks for your response! I'm disappointed that FileMaker is missing something very basic and I hope to see it in the next version. If anyone is curious, I have gotten around this issue by using the 360Works ScriptMaster Extension (http://360works.com/scriptmaster/). It has the ability to browse to a directory and get the directory path. The downside is that I now have dependencies when deploying the application (more failure points).
You have very succinctly but effectively summed up the pros and cons of plug ins.
Best practice is to not use them unless you truly have to even if the alternative is a bit cumbersome. In your case, I'd probably go with the plug in. But it may be some comfort to know that 360Works has been around for quite a while and thus is a pretty "good bet" that they will remain and continue to support their plug ins as new FMP versions get released.