Whiskey and bourbon are aged in oak barrels, then some barrels are given a second life at wineries or rum distilleries. The aging process is dependent on temperature swings and the alcohol exchanging thru the wood.

My question is what would happen if a charred piece of oak (same species as barrel oak) were put in a bottle of grain alcohol and allowed to set/soak for an extended period of time, and allowed to go thru temperature swings? Would it become whiskey in a sense, or just brown/gray grain alcohol?

Grain alcohol is too hot and in my opinion not a good mash bill for aging and trying to create a decent sipping whiskey.

Start with a "white dog" (Evan Williams etc.) or distill your own mash then you can do the "charred oak stave in a glass jar" trick if you want to hurry the aging along.

This^^^

Oak & Eden finishes in the bottle in a similar fashion.

You also want the ABV during either barrel or charred stave of oak to be  60-65% with 62.5(125 proof) thought to be optimum. After aging, it's gets cut to 80 proof.

https://thewhiskeywash.com/lifestyle/7728/

Whiskey starts its life as grain alcohol, so there’s that technicality.  The specific requirements for certain whiskies are mainly what sets it apart from neutral grain spirits.

But if you’re contemplating taking some higher proof vodka and adding some oak (spiral, chips, etc), it will taste like shit.  I’ve tried several variations of this, up to Buffalo Trace white dog aged in a real barrel, and it’s still awful.  Way too much quality whisky/ey on the shelf to make this experiment worthwhile.