Interesting. I've never heard the phrase "joyous dinner attire". It sounds kinda nice, though Looking at the examples on Angie's cheat sheets, I feel that the setting of the farm trumps the evening time in terms of what style of outfit to pick. I'd consider the dresses pictured in the "Summer Day Wedding" appropriate, and would think that the dresses pictured in the "Evening Wedding" are overkill.
I went to a summer wedding at a farm last year. I wore a knee length A-line dress with white background and a green, red and fuchsia floral pattern, and brought a cardigan (which I definitely needed later in the evening.) Figuring out the shoes was indeed the hardest part, especially since I knew there'd be contraband dancing at the reception. In the end I wore my red strappy sandals for the ceremony, and then switched into flats for the dancing. THere were a couple other women who brought an extra pair of shoes, too.
The wedding I went to was on the more casual end of the spectrum, but even for a more formal event I'd go with the same style of outfit. I'd choose a fancier dress in terms of the material and details (mine was a relatively plain cotton in a simple cut; for a more formal wedding I'd look for something with more shine or flounce or interesting details to bring it up a notch, but I'd still stick with the A-line and knee length).
Hope this helps. You might want to ask the couple how formal/informal you should interpret the dress code. My hunch is that "joyous" translates to less rather than more formal.
