Does Craft enforce a maximum filesize for large files? A client we're working with has a requirement to upload one large file (about 320MB) every 2–3 months. Craft has been configured with a local asset source to handle this.

I'm trying to decide the best way to approach this: whether we upload this large file via SFTP for them when required, or if the client can do this through Craft in their own time.

This previous question provides some good pointers. I've increased the necessary settings, however when attempting to upload a test file (315MB), Craft displays an alert ‘The file “testfile.zip” could not be uploaded, because it exceeds the maximum upload size of 128.0 MB.’

I haven't seen any filesize limits of 128MB, so is this value hardwired into the file upload logic? To confirm that the changed settings do work, a test file of 100MB was uploaded fine.
The only thing Craft enforces is the maxUploadFileSize config setting, which by default is set to 16MB.  Outside of that, any limitations you run into will be environmental/server config related.