Mixing GPL and non-copyleft code in source files

[Wojtek Kosior via "Development of GNU Guix and the GNU System distribution." <guix-devel@gnu.org>]

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Hi

I have some changes in my personal Guix fork that I'd like to
contribute.  I'd like the lines of code I authored to be dual-licensed
under GPL-3.0-or-later and CC0-1.0.  This is purely for personal
reasons (as having a mere few dozens of lines in a project under CC0
causes no practical change to the project).

How can I indicate such licensing?  In one patch I sent 5 months ago (a
still unreviewed one, unfortunately) I added a line like

    ;;; Copyright © 2023 Wojtek Kosior <my-contribution-is-licensed-cc0@koszko.org>

while keeping the rest of the license notice intact.  Yup, this happens
to be an existing email address that also conveys a message.  However,
I am worried this might be too ambigious.

Are there better ways?  How about introducing an SPDX license
identifier + explaining the licensing situation in the commit message?

Btw, is there anything that from technical side should be corrected in
the linked patch?  I mean, besides my failure to use `list` with gexps
for the `arguments` field (I already realized that shortcoming myself)

Wojtek

[1] https://issues.guix.gnu.org/64869

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