Set a mutt configuration value based on the output of a script?

Question

I've found a bug in mutt, but would like to build a workaround until it can be fixed.

~ (the mark-message function) is supposed to assign a shortcut (i.e., a macro) to mark a message so that you can jump to it later. For instance, if you mark a message with the shortcut d, mutt will automatically set the following macro:

macro index "'d" "<search>~i '[Message-ID Header]'" "message hotkey"

so that you can hit 'd to jump back to the message you marked.

The problem is that the Message-ID header is interpreted as a regular expression, and sometimes the headers of messages I receive contain special regex characters, like +. (See example below.)

<[e-mail]>

If they do, then mutt fails to find the message, and so the macro doesn't work, and simply throws the error Not found..

The solution would be to backslash-escape those special characters, but I don't believe there's a way to do that natively in mutt. What I'd like to do is basically build my own substitute for the mark-message function. I'm thinking I'd need to pipe the given message to a script, grep and cut to extract the Message-ID header, and then sed to backslash-escape any special characters.

The problem is trying to pass this string back to mutt: I know you can use environment variables in muttrc, but I believe those are evaluated upon assignment, and not on execution. Is there any way to get the string from a bash script back into mutt's configuration?