Gentoo like use-flags implemented as an experimental guix third-party lib?

[jgart]

Hi Guix! I have a proposal:

It would be cool to offer gentoo like use-flags for guix as a third-party guix 
library.

See here for attempts in nixpkgs to get something similar upstream:

https://github.com/NixOS/nixpkgs/issues/12877

https://discourse.nixos.org/t/gentoo-like-use-flags/2210

I think this would be a perfect use case to put in practice the guix thesis and 
to showcase the freedom of extensibility it offers users without disrupting 
upstream project directions. For example, the guix-use-flags library could be 
offered in a guix channel.

guix-use-flags would allow users to add global filters to packages triggering 
rebuilds under those filters (e.g. disable sound, disable gui/xorg across all 
packages, etc...)

USE="ldap mbox sasl imap apache2 -png -jpeg -X -kde -gnome -gtk -gtk2 -gif -qt 
-opengl php -truetype"

would translate to a guix api as:

(use-flags '(ldap mbox sasl imap apache2 -png -jpeg -X -kde -gnome -gtk -gtk2 
-gif -qt -opengl php -truetype))

I imagine this can be a global configuration setting that could be implemented 
via macros to rewrite (package ... '(#:configure-flags ... with the appropriate 
flags across all stated packages in a system configuration.

I think this would be especially useful for configuring guix system servers.

What are your thoughts on this? Is there an alternative way to do what I have 
described above that I am not thinking of?