Matt, you should come to The InDesign Conference next month in New Orleans… there are some attendees coming from Wizards of the Coast, as well as a whole session on using anchored objects. That’s probably what you’re going to need here: anchored objects.

InDesign doesn’t do colored font characters, so the font trick would work if it’s just black and white, as you discovered.

If it’s coming in as text, then there are various ways to swap the text with images, such as:
https://creativepro.com/find-text-change-to-graphic.php

Or there are some scripts like this one:
https://www.rorohiko.com/wordpress/2014/06/24/peoples-choice-indesign-script-pepcon-2014-replace-words-images-vice-versa/

OK, wait… why on earth would you need to put a space between every 7th digit? Not every 10th digit? What mathematical planet are you from? ;-)

And why pi?! (You may or may not know that the co-host of this web site — me — is also a pi fanatic.)

Here’s one solution: In the find/change dialog box, choose the GREP tab. Then search for (\d{7}) and replace it with $0

Then click Change All. That will insert a space after every 7th digit.