Unfortunately v4 and v5 and v7 mat files were not defined to allow for comments. The exception is that the first 128 bytes of v5 and v7 files is text and that everything after naming the version is ignored, so about 110 bytes are available.
In theory Mathworks could extend the v5 and v7 header to include a comment marked as such, but doing so would cause backwards compatibility problems.
v7.3 mat files are hdf5 files, and that standard has no provision for file comments. The standard does have an Object Comment that can be applied to each data object, but there is not one for the file level as far as I can see.
I suspect that the easiest approach would be to designate a particular variable name to store in the file as a file description, and to write a small wrapper that checks the file for that variable and pulls it out and displays it if present. You would need to decide between char() vs string() vs cell array of character vectors.
