The game doesn't recognize "having it naturally", only "having it".
It is coded solely for Castform.
The way Forecast works, it's not an option.
It will only function when given to Castform.
Unless you feel like changing the way the ability functions, you're out of luck.
That being said, the same thing applies to Arceus and the multi-type ability in that it only works with him. Programming should be a non-issue, however it does seem that it would clearly alter something that was meant to be exclusive and non-interchangeable. I suppose would be up to the community to decide if there is any good reason why said abilites should be mutually exclusive. Personally I think Multi-type would allow for a pokemon that could fill any positon. While it might be somewhat interesting, I think it would be too broad. The same could be said for forcast, but to a lesser degree.
-Filter is the same as Solid rock, which is on an OU pokemon.
-Unaware, Frisk, Compound eyes, Shed skin, Rockhead, Battle armor,Vital spirit(Same as insomnia), Levitate, and Limber are on previous CAP's and could possibly be avoided for this CAP.
-Huge Power, Pure Power, and Wonder Guard seem a bit over powered.
IMO these seem to be the most general and balanced underused abilities:
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Leaf guard prevents all status conditions in sunlight and seems like it could be fairly interesting for any team that takes advantage of sunlight.
*Magic guard prevents all indirect damage (stealth rock, spikes, poison burn, sandstorm, hail, Life orb recoil) and seems like a very solid basis.
Marvel Scale Increases defense by 50% when statused and could be the solid basis of a stat absorber.
*Mold Breaker Negates any abilty that hinder attacks(Filter, Solid rock, Damp, Levitate, Heat proof, Thick fat etc.) and would be very useful to any sweeper.
Shield Dust Could be used on most anything, and blocking secondary effects is always a plus.
*Tinted LensCould be an absolute beast. This would allow for decent damage to be done to nearly anything.
Just my two cents.