External reviewers of performances or displayed art can be one of the best ways to document the quality of a performance or displayed artwork. The faculty member him/herself should not be involved in obtaining reviews. The department head should be the one to work with experts in the field to obtain reviews.
Please be sure there is no conflict of interest between the expert and the faculty member. Treat these like you would treat the external review letters.
If there is someone who is not close enough to come to an exhibition or performance consider sending material to them. Musical performances could be recorded - make sure you are recording them with the best possible quality. Think about whether there is any visual element to the performance. In evaluating the director of a musical group, the sound is fine but the directing is an important part of the performance so a video recording would be better than just an audio recording.
For artwork, pictures of an exhibition may not be enough for an adequate evaluation. If it is possible to send pictures of the exhibition as well as one or two pieces of the art to the person, that might enhance a review (after the exhibition is over, of course).
For theatrical performances, video is the obvious choice but if the set is the primary aspect being evaluated a video of a performance might not be enough. Detailed still photos, plans, blueprints, etc. might also be required.
In short, if a reviewer can't come to you, send everything that might help the reviewer adequately and fairly review your work.
Kloth (1991) noted that review of theatre productions are best done if the review can be involved throughout the production rather than just reviewing the final production. Since one aspect can affect others, it can help the reviewer see the production as a process, not just a finished product.
Kloth, K. (1991). Problems in recognition of artistic and creative work in academic theatre departments. ACA Bulletin(75), 52-62.
If artwork or a performer (or group) are invited to perform at an event, the letter of invitation could be used as evidence of quality. For instance, the Pride has been known to perform at the Rose Bowl parade and at presidential inaugurations. Neither of those opportunities is open to just any marching band. These performances themselves are evidence of quality. Be sure to include a description of how the band, artwork, or other artistic group/venture was chosen to perform or be shown.
If a piece of art, music, or a performance wins a contest, that win is evidence of quality. Make sure the criteria for the contest is adequately explained so those evaluating the evidence know what was expected, how many entries there were, etc.
Art exhibitions at art museums or gallaries when the exhibition was solicited by the museum or gallary is also evidence of quality. Include these invitations as evidence of quality. If you are invited verbally to provide an exhibition of your work, ask if you can get an invitation in writing and use that as evidence.
As with all evidence, that listed here will be stronger or weaker depending on where the invitation comes from or who you are competing against. Make sure that you include in your evidence enough explanation so that those who are not familiar with art, music, theatre, dance, etc. can tell how important that invitation or contest was in your area. E.g. having the Pride asked to march in the Brookings 4th of July parade is not very prestigious but being asked to play at the White House at the 4th of July celebration is.