Hi. I don't know about proof in terms of joining DAR - but if they have accepted members in past who are your documented ancestors then it seems you would be good there.

On Ancestry.com they have scanned thousands of SAR and DAR membership applications. I found one connected to someone who is related to me and I wrote this piece:

https://medium.com/age-of-awareness/is-my-genealogy-hobby-racist-9bdf6df6d60c

I personally think you don't need to prove anything in order to write about DAR, but it might add a little pizzazz to the story to have documented ancestors. If you don't have an account on Ancestry, I can do some digging for you and download documents if I find them.