I have tried to find a way to show custom headers when reading mail using Emacs and mu4e. Unfortunately, I never did. However, I find a way to show arbitrary custom headers in Thunderbird, which is fantastic! In Config Editor, you can add a list of headers separated by spaces to `mailnews.headers.extraExpandedHeaders`.
Why would I want to do this? you may ask. I use a Postfix virtual table with wildcards to allow me to set up email addresses on the fly, and I use sieve to evaluate the detail part (everything after the plus in a plussed email address) and insert a header identifying the original company. This is really more of a nice-to-have thing, but it's also very helpful to see how a given email address is used.