From the main gallery page, I'd then like to be able to filter by this same custom field. (It would look and function in a similar way to the built-in "tag" system, though the selection of "tags" in this proposed system would be limited by the custom field options defined by the administrator).
I'm prepared to pay to fund the add-on's development, and you may keep the rights to publish or sell the add-on for others in the community as you wish. I have seen similar feature requests by other users in the past (including this one, by @Billyhoyle), so I do believe there would be some demand for it.
If you'd like to take this on, please start a conversation with me which includes a cost quote. Thanks!