I think most of the time if they have a Github/Gitlab repo open to the public opening issues, they will accept an issue that merely describes a problem that needs to be fixed along with how it might be fixed.

Something like this is generally appreciated:

I'm an HCI expert and ____ should be improved to do ____ because of ____. I'm willing to volunteer to do design work on this to help the project out

...and if the maintainers ask for contact info, provide it and there you go.

there is never even an opportunity to submit design work.

nowadays, design work is code (or at least markup). Like .qml or .glade or even html/css.

is asking people who don't typically code to do so

encounter a problem and learning a new tool to solve it, is a pretty standard situation in IT. Designers have varying capabilities in that domain. While a print designer might have trouble learning a new software, a MMI-Architect or UI Designer should commonly be able to learn a new UI framework (designer software).

Generally, if you like to contribute to any project (not limited to software), you can't expect the project to adapt to your toolset.

But often it's enough to share an idea for it to get adapted.