I just had a gig like this! A friend of mine is heavily into MWSD and is always looking for ways to cross-pollinate the scene. She asked me to come and share an evening with the regular caller because they couldn't get a round dance caller for that particular night. I did not have a live band; I used recordings, and I provided my own sound gear and mic.
Here's what I learned:
1. Long swings, of even eight counts, are not intuitive and you have to spell out exactly what you want. The one time I introduced an eight count swing, I counted it for them so they knew how long it was going to be - and some people did not care for it at all. It does mean that a lot of the easy contras I have, with 16-count swings that give people a chance to recover and get back on the phrase, do not work very well.
2. Many calls overlap, and you can use those to your advantage. I was trying to figure out if I was going to have to teach a ladies' chain, for example, and I asked a MWSD caller acquaintance what would happen if I said to the group, with no explanation, "Ladies, chain across the set," and when I realized it was exactly what I needed them to do, I relaxed a little. :)
3. Be prepared for something you say to *not* mean the same thing you intend, even if you think you are being exceedingly precise. I can't remember what it was - but I intended for people to end up back to back with their neighbor, facing a new foursome. What I got was people standing shoulder to shoulder with their neighbors in what a contra dancer might describe as a "short wavy line" without hands.
That point about becket dances perhaps being easier to understand progression in is something I noticed at another recent gig I called - interesting enough that it is making me rethink what I have flagged as "first three dances of the night."
Good luck and have fun!
Meg