The Johto Safari Zone actually seems to offer more than currently known, and many currently labelled unobtainable Pokemon are actually obtainable. There appears to be further gimmicks that the items you place in each zone work, which could lure out even rarer species to each area.
We still don't fully understand how it works yet, but it involves a 'levelling up' system in each area and their placed items, which has something to do with time. I placed 30 grassland items in the Marsh area, and one day later, I encountered a level 47 Seviper, which is up til now, considered unobtainable in HGSS.
Many more testings are needed, but if this all works out, rare species including the Bagon line, Beldum line, Gible line and Riolu lines are all obtainable in the Safari Zone.
This makes the true unobtainables in HGSS much less, mostly the 4th gen starters, the other legendaries, and several Sinnoh exclusive lines like their own fossils, the Stunky/Glameow lines, Drifloon line and Rotom. Of course, we can't get Leafeon, Glaceon, Probopass and Magnezone as well due to the evolution area. Other than these, all seem to be in.
Coronis~
