[silotis]

I seems to me best approach would be to compress the contents with a Huffman code or some other entropy encoding. All this business of restricted character sets is just an ad-hoc way of reducing the size of each symbol and we've got much more mature solutions for that.

[account42]

For entropy codes to be effective for such short strings you need a shared initial probability table. And if you have that you are effectively back at special encoding modes for each character set.