I feel like I should be able to do this, but if I want to create an html block that displays on all Custom Certificate activity pages in all courses - How would I configure this? After testing all the options I haven't accomplished it:
all activities or all earned certificates?
On all Custom Certificate activity pages in each course
How many certificates do you have in your courses?
I think there is a block that goes on the Dashboard that shows all the certificates students earned, generally. I'm not sure if it is a plugin or not.
You won't be able to use the 'HTML' block that ships with core as it will require accessing the database and some PHP code to display the activities which you certainly can't do via the 'HTML' block (that would be a huge security hole if possible). This would require you to create a new block. It wouldn't be much code, perhaps there a similar block out there.