Hello Jonathan,
Would you consider bedding the joint with a bedding compound? Shipwrights use dolphinite, but it canrun out in the first months with heat if the joint isn't especially tight. It tacks up harded over time. Shipwrights also use a mixture of turps, bees wax, and pine tar. I can't find my recipe right now, but if your'e interested I can keep looking.
I also bed all my nonvisual joints in tar, such as post bottoms to bracket. Shipwrights call it bear ____, if you want to talk the lingo.
My experience with red cedar, and it's much wetter up here, is the wood holds up well if you can prevent water from entering any end grain. There's always Anchor Seal for that. Unfortunately, it runs like crazy on visual stuff, but once it's dry it's fairly stable. And you can recoat when it's tacky but not completely dry.
Best of luck.
Zach
