You don't need ASM to edit the encounter rate, it is literally a one byte change. Also, once the Shiny is caught, it's permanently shiny, meaning if you put a hack that had the hex edit on it onto a cartridge and caught a boosted rate shiny, it would trade over and still count, even if the shiny rate is lower in the recipient game and it wouldn't properly be shiny there. From there you can trade it up and it will still be legal and still be shiny, however if you wanted to do that you could just use a cheat code lol