You could do the same spitting trick but only split at progressively increasing file lengths at the character '5'. The "compression" would be worse, so you'd need a larger starting file, but you could still satisfy the requirements this way and be independent of the filenames. The decompressor would just sort the files by increasing length before merging.

Nice idea, but doesn't this require a linear increase of the length of the partial files and a quadratic size of the original file?

If the length of a file is X, then in the next file you must skip the first X characters and look for a "5" that in average is in the X+128 position. So the average length of the Nth file is 128*N and if you want to reduce C bytes the size of the original file should be ~128C^2/2 (instead of the linear 128*C in the article).

That's a neat idea.