My neighbor is installing some 6x6 treated posts for a customer. She wants to stain the posts and they have some large cracks that need to be filled. Is there a stainable exterior wood filler?
I'd be more concerned that you can actually keep the cracks filled. Wood outside will expand and contract so much that these will likely open more over time.
Search log home caulking and you will find what you need. I used a caulk called "big stretch" on my log home with great results and it comes in a variety of colors.