Thanks. I am not sure how I missed that section in the manual that answers my exact question… I will give that a try!
Once you apply the override concept with Weechat, you'll have a feel for what is a very general suite of customization mechanisms in Nixpkgs. That'll be unlikely to miss a similar opportunity with other packages.
NixOS Discourse is a great place to clear stumbling blocks like these, by the way :)
Thanks for convincing me to give it another shot!