Hi John,
You probably know that Hal Landen has a mizzen stays'l for #18 Satin Doll. I think you have the link to Hal's site on your blog:
http://www.videouniversity.com/sail.shtml.
As Robert mentioned in the post of his old boat, he installed a mizzen mast abaft of where your mast and mine are located. I'm not sure how precise you need to with the measurement of a stays'l, but Robert's sail would probably be a little big for our boats. I attempted to derive the measurements of the stays'l in the sail plan you show above and here's what I got:
25.8' (25' 9") luff, 20' foot, 18.3' (18' 4") leach
I also hope to order a kit from Sailrite one day but first I'd want to make some measurements on my boat and then maybe even mock up a sail out of a tarp or something similar. However, if you do it first, let me know! I also discovered recently that a previous owner installed a mizzen sail with a longer luff so that the boom no longer clears the back stay. While the mizzen sail would be furled when I fly a stays'l, I'd have to think about the boom height when sizing a stays'l.
BTW, hope you're enjoying #15. I sailed on her in Narragansett Bay when Denis had her - the only other Triton yawl I've been on.
Mark