I have made a muscadine pyment, but I did not use fresh grapes. I was able to purchase muscadine juice from a winery in Florida (Dakotah Vineyards & Winery), and I used 1 gallon of juice added to a 5 gal batch of already fermenting mead. I did it this way b/c the juice does have some sulfites and citric acid, so I wanted to have an actively fermenting yeast just to make sure that the preservatives wouldn't affect the lag time. It came out really good, if I do say so myself.
If you have a press, I'd say you're golden. I do remember thinking that I might consider adding up to 2 gal of juice if I did it again, so I'd say anywhere between 1 and 2 gal of juice and water/honey up to a total volume of 6 gal would yield a great pyment. I used 15 lbs of honey for the 6 gal batch.
Packaged: Dark Munich Citra SMaSH, Mayan Stout, Caramel Quad, Basic Spiced Cider, Spur of the Moment Graff
Recent Meads: Mead Day '11 Ginger Metheglin, Mead Day '12 Traditional (orange blossom) Mead, Fresh Simple Cyser '12, Cherry Melomel, Belgeglin, Bochet
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet
Planned: Hop Metheglin #3 (NZ hops), Trad. Gesho T'ej