McCloskey Man O’ War marine spar varnish is available in satin finish, and is purported bo be tung oil based and “Superior to polyurethane because it is flexible enough to expand with the wood.”
Some time ago i ordered a couple of quarts of True Tone satin spar varnish from McClendon's hardware in Renton WA. I use it over Epiphanes as a final coat to cut the gloss. It's been on several years now and still looks great.
I tried to find a website you could order from but had no luck. I believe I contacted them by phone. Here's a link to their contact info: http://www.mclendons.com
The product is fairly thin and is relatively easy to work with.