The default rule tile doesn’t have an option for displaying text in the footer.
You could use a Super Tile if you wanted to customize the type of content and layout of things. That’s a great approach for a few one-off tiles, but probably wouldn’t scale well if you have lots and lots of rule tiles you want to make that type of customization to.
One of the neat use-cases for a Super Tile is combining multiple items together. For example, I sometimes use them when I want to have a list of hyperlinks or dashboard links on a single tile.
As you alluded to, you could use a Custom Tile for this, but it would require some developer skills and you would likely need to add an HTTP Trigger to each rule you wanted to trigger.
Alternatively, any of the tiles that support Hyperlink Actions can use the special $.runRule("RULEID") hyperlink syntax to run a rule.