-Single reel or Multireel by content:
Use the original media file as the filename.
-Single media multireel by size:
Use the original media file as the filename, plus the reel number, plus the total number of reels.
-Multiple media and multireel by size:
Original media file by the start of the reel. Plus reel number, plus the total number of reels.

Exemple:
j2c_original media_mov_001of004.mxf

UUID can still be added if it's useful for machine processing or other.
The filename length limit for the common filesystems is 255 characters.

In a similar way, Annotationtext can have more human readable information. Right now in PKL they are a mere duplicate of the Asset Id.

Some Annotationtext are indeed a duplicate of some kind to compensate errors of other software. Notably Doremi CinePlayer using the CPL reel Annotitiontext as the path to the file.

Are you saying that CinePlayer must have the path to the file in the AnnotationText?

Are you saying that CinePlayer must have the path to the file in the AnnotationText?
My bad, apparently is the <OriginalFileName> field, it's a known error that looks like it has never been solved on CinePlayer. I think it was added in DCP-o-matic at some point, this is the only reference I found http://dcpomatic.com/forum/viewtopic.php?t=95&start=10

Maybe the <OriginalFileName> field is the right place for the source filename? It's not exactly the same. And the naming template can have other uses. This solutions are not mutually exclusive.