I mean there are probably a few bits you could scrape from the lock time value but that'll also be very limited and it'll have to be a block height that has already passed...

20 bytes is what would be needed to store unique identifiers on the chain afaik so it's what you're going to have to put up with... But why you think others should download whatever junk you dump on there confuses me... Try it once or twice don't use 12+ kb of space that no one else can use for anything...