There is a process described in TiHKAL for oxidizing harmaline to harmine using nitric acid, which is not kitchen-safe.

Harmaline also oxidizes on its own, but it's a slow process. Is there a low-tech, safe way to speed it up?
Temperature, sunlight?

For now, I just store my harmaline as freebase at room temp to promote oxidation (as opposed to the THH, which I store in the fridge as a salt, to slow down oxidation).

Any idea roughly how long it takes for harmaline to oxidize in these conditions?

I wish I could answer this question in some useful way.

Shulgin mentioned (I think) Richard Spruce's hundred-year-old sample of caapi that was analysed and found to contain exclusively harmine, so the upper bound can be set at 100 years or less. Clearly, we want a more favorable timescale for this reaction and as far as deliberately carrying out the reaction goes, we may need to do some experimentation.

An unreliable imp at the back of my mind says manganese dioxide may be worth a try, possibly in the form of Carpino's reagent. Otherwise we might want to look at some form of catalytic dehydrogenation with a platinum group metal. It all gets a bit technical here and OTC kitchen-friendliness possibly goes out the window.