I've never tried FR/LG. I thought Channel Jirachi was impossible to RNG though... I guess "impractical" is the better word, since not only is your game saved before you can check your result, but you have to beat the entire game again on a new file to try again.
Is RNGing the WISHMKR Jirachi technically possible as well? At least you only have to get the Pokédex on a new file to try again.
I edited the seed on my Wii and then sent over the Jirachi to my copy of Sapphire version. It's not Smogon legal due to the hack to edit the RNG state. I'm not sure if changing to code to edit the RTC for me would actually make it legal or not but I don't quite care that much.