@woutertron is there any chance that you use Discord chat, or would be willing to?
There is a libretro/RetrArch Discord server where all of the active mame2003-plus developers (as well as a lot of other RA and core devs) can be reached. This is a direct link to the Discord channel, where I can be PM'd or tagged in #programming: https://discord.gg/C4amCeV
The other way that mame2003-plus developers tend to communicate is via github. It so happens that grant already has an issue stubbed out to talk about documentation related to input and input remapping: https://github.com/libretro/mame2003-plus-libretro/issues/329
That would be a great place to start if you're interested! :D