I think it would be interesting but I see no good reason to do a retreat for specifically for rationalists. I'm not sure whether there are people in this community who are at the 10,000 hour skill level. It's probably easier to find that outside.

I'm apologizing in advance for a bit of bit irreducible spirituality. I try to keep it to a minimum.

I have two experiences of group meditations among rationalists. On was last year at the European Community Camp in Berlin. It felt like there was nobody in the room who gave the thing stability. In the dose of 15-20 minutes that's not harmful but I wouldn't expect a few days in that state to be good.

I lead a meditation during the solstice celebration in Leipzig and only used 20 minutes of 30 that I had asked for on the agenda, because that felt enough.

In total that leads me to think that meditation makes a decent agenda item at an LW event but no good program for a complete day. At least without someone to lead the event who really knows what they are doing.