I think this is a retreat issue, not a temp instrument issue.

Because-- I would be very surprised if a retreat billed as "silent meditation" will allow the guitar. Best check with the facility. Also others may also be there on a similarly silent basis and not appreciate the distraction of your particular regular practice.

To maintain calluses of course you can finger any instrument, and just not strum it, but a retreat is, by its definition, an opportunity to be without something one usually does.

(PM for more info if desired.)

~Susan