Is it theoretically possible to get any shiny IV combination for eggs (provided the IV spread is found in the egg IV portion of the Time Finder) with any TID/SID in Gen IV? I'm asking because I RNG'd my TID/SID so 31/31/31/31/31/31 Modest and Calm were shiny spreads for wild and gifts, I don't know if that's how eggs work or not. I'm just starting to look at egg mechanics and wanted to know.
Yea, it is. Egg abuse involves two different RNGs: the first one happens before the egg is generated at the Daycare and affects the egg's Nature, Ability, gender and shininess. After the egg is generated, those aspects are
locked (every time you hatch that egg, it will have the same Nature, Ability, gender and shininess, provided, of course, you save after generating the egg, before taking it).
The second abuse affects the IVs. This involves advancing the frames and taking the egg from the Daycare.
Since those are unrelated, and your ID/SID combination only affects the first abuse, it's possible to get any combination you want. (Of course, the egg's IVs depend on the parents' IVs)
tl;dr - yes, it is possible. :)