Only 50mm of the seat post is supported and the lower part of your top tube/seat tube joint will be on the thin part of the tube. How big of an issue will depend on how far the seat post extends out of the frame. The potential for cracking at the ST/TT joint is probably minimized by the fact that this is a lugged frame. There still could be some seat post creaking issues with the minimal amount of support and the end of the post moving around in the butted area when the post flexes.