I suggest starting with a theme component here prior to considering any core changes.
Sorry for the delayed reply. I have always regarded Discourse development as both thoughtful and thorough, and my perception from other threads as also led me to believe that accessibility is a priority.
Which is why I am frankly struggling to understand the reluctance here. I would describe the current situation for users as “nearly impossible to add, even if they want to / remember to” and the situation for admins as “completely impossible to remind / encourage”. Making alt-tags simpler and easier seems like incredibly low-hanging fruit that would benefit accessibility on every Discourse instance. It should be built-in, everywhere. $0.02