i just was soft resetting for like a full day as of now and i just got me a shiny dialga from my Heart Gold version! i thought i'd share the info with the community because i didn't see it posted anywhere around here that the dialga, palkia, and giratina can be shiny. The one from the Arceus event from jp.! Screenie of the Dialga :
so yes, the dialga, palkia, and giratina from hgss can be shiny. Mine actually ended up with overall excellent ivs and a decent nature. (30 IVs in defense, Quirky Nature) it took like the whole day today to finally get it but i did eventually get a juicy reward out of the time spent on getting it.