To my knowledge, "Denatured" alcohol simply means alcohol that has been made undrinkable for recreational purposes.
If by 99% IPA you mean 99% Isopropyl Alcohol, rest assured, it is definitely denatured!
If you are referring to common 70% "rubbing alcohol", it is usable for making case lube, but be advised that it will not be a very satisfactory substitution.
The alcohol in this instance acts as a "carrier", its only function is to dilute the lanolin, allowing it to evenly coat your cases before quickly evaporating away.
If your alcohol is 30% water, it won't evaporate quickly and won't evenly coat the cases with lube.
It's not the cheapest way to buy it, but Iso Heet gas drier from your local auto parts store works very well and evaporates quickly.
Make sure you get ISO Heet and NOT regular Heet.
I would strongly advise that methyl alcohol NOT be used in anything that will likely be in contact with with any liviing thing that you care to preserve.