During previews when we have daytime tech rehearsals, I create a daily Rehearsal/Performance Report with a section for rehearsal, a section for the performance, and a section for technical notes. I think I would do the same for an Understudy Rehearsal day. Or, just a note in the General/Performance section saying a rehearsal was held, for what parts of the show, and how long. I wouldn't include issues about understudies' performance in a full-staff report, only in a direct email to Director/Artistic Director as appropriate.