Maia,
Let me see if I'm correctly reading your goals:
You want to call dances with shadow swings
You want to minimize discomfort
To do both of those, I think an announcement after folks have lined up is worse than an announcement earlier. If someone is uncomfortable with the idea of swinging a shadow, how comfortable would they be leaving a line after you've made the announcement and highlighting for everyone in the room that there's a problematic interaction? There's no way to surreptitiously drop out; folks have to take new hands four and identify new shadows. I think making the announcement after folks have lined up just puts public pressure on your dancers.
You can announce it ahead of time, either at the end of the previous dance; or even during the walkthrough of the previous dance. I know some callers do this for mixers: "Alright, thanks everyone for lining up and taking hands four. Just so you know, the dance after this will be a mixer. You'll keep your partner for this dance, but the one after don't expect to stay with your sweetheart." etc. Andrea raises a good point that it's hard to know where the shadow would be when you line up (or if someone lines up after you).
Other options that come to mind:
- Call these dances that are dear to you only in instances where it's unlikely to be an issue. I.e. small dance communities where you know everyone and know it'll be fine; or very large events (dance weekends, etc) where it's much less likely for you to run into someone you have a bad interaction with. If you had a festival session called "Shadow Dances"; then anybody showing up is probably going to expect that they'll swing the occasional shadow.
- As Ron suggested, you can modify the dance and say "and this move can be a swing". Might defeat why you're trying to call the dance though.
- Make it triplet-esque; in that rather than full long lines, you break folks into small groups (10 dancers, etc) and run the dance for a short time. If there's a shadow, partner, and neighbor swing, chances are you don't want to run the dance super long anyway. You announce "find a partner and about 4 other couples for a special dance" and folks can self select a little more. Smaller groups also let more experienced dancers goof/chaos/play more with repetitive dances without throwing off large portions of the hall (I consider a dance with shadow & partner swing, with or without a neighbor swing; to have a high possibility of repetitiveness).
Hope that helps get you thinking about other options too. Have fun, and remember we're there for the dancers to have a good time.
Luke