I'm no septic tank specialist by any means, but thinking over your question I'd have to say no good could come by passing 15-20 gals of fresh water through your tank every day for months on end. A holding tank is just that, a holding tank. Constantly flushing that tank out with fresh water would have to decrease the friendly bacteria levels and therefore decrease the effectiveness of the holding tank to treat grey matter.
As far as plumbing your A/C discharge into your garden for watering, I like that idea. But, you should have some sort of diversion route in place, "most" plants Don't like to live in basically standing water......15-20 gallons a day is a lot of water! Now if you were to make basically a bus route of sweater hoses throughout your whole garden with gate valves to direct water flow PLUS having some sort of free flowing discharge I think you could be the talk of the town.
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Scotland, Ontario Canada 
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