Add command-key option to dwm and dmenu

What do you mean?

If you meant to ask, whether this works, then the answer is Yes.
if you meant to ask, whether there is a way to do this using the dwm build system, then the answer is No. Dwm is directly "configured" at compile time by editing config.def.h.

Speaking of compile-time configuration: Is it possible to pass arguments to the brew command line options?

In order to personalise dwm a `--dwm-config=$PATH_TO_CUSTOM_CONFIG.H` would be great, then the formula could copy `$PATH_TO_CUSTOM_CONFIG.H` into the source tree and build dwm.