Hey Jacob,
We hadn’t really considered that yet. The way the search currently works is by matching post titles to the search query. Are you suggesting that the search could match custom field values, or custom field keys?
Either way, adding that kind of data to the search index would increase the cache size dramatically. However, we are planning on moving our search functions to use the WP REST API in the near future, so perhaps searching by custom fields in addition to post title could be a feature for when we work on that.
I’ve opened a new issue for this on the ACP Github repo, so if you have more comments/suggestions for us, feel free to add them there. Thanks for the idea!