Every so often the topic of adding a scripting language to Mutt comes up. Most people don't understand the issues involved, and why this hasn't been done so far. It's not a matter of the developers being lazy, it's just a very difficult problem to solve aesthetically.
Probably by far the biggest issue would be of chosing which scripting language. S-Lang is often suggested since Mutt can already be linked against it to use its screen management capabilities in place of ncurses. The problem is that everyone has their own favorite scripting language (eg. Perl, Python, TCL, Scheme). If scripting support were ever to be added, it would probably follow the model used by the GIMP, with its procedural database (PDB) that allows you to write extensions in any language, and make it available to any other script no matter what language.
Engineering-wise, adding scripting to Mutt at this point is difficult. If you look at other applications with scripting support, primarily editors, they all operate on one basic mode (a buffer). Mutt on the other hand has several different modes--menus, pager. Not all functions can be applied to every mode (consider Emacs: even when you have different major modes, there is still a concept of a cursor-up or cursor-down operation). So each instrinsic function (the builtin functions available to scripts) would basically have to keep track of what modes it is allowed to be called from--messy. (ie, you can't call group-reply while in the file browser because that wouldn't make sense).
For the most part, the keyboard macros in Mutt allow you to do most everything you need to do. There are only a few places where scripting support would really make sense.
- prompt in the middle of a macro is impossible
- applying a macro to all tagged messages
- arbitrary editing of the message header in compose mode
Complex motions can be accomplished already, so they are not included here. Some ConfigTricks might guide you in the right direction, especially for "if" control.