I'd love to attempt one of those cake cocoa mugs, but how do I affix the MMF handle to the cake? Seems that even with "glue" the handle will be too heavy...
You are right it is too heavy.
Don't think of it as a handle--think of it as an attachment to the cake that the part that sticks out looks like a handle and the part you can't see keeps it securely affixed inside the cake. Often the part you can't see is bigger than the part you can see. Kwim?
Candy clay is a nice medium to use also.
Maybe you could use floral wire coverd in MMF?
What I'm trying to say is weight this in your favor. Don't make the handle part the heavy part--it will fall out. Make it smart so you can stick it in the cake and it's own weight and design will just keep the handle showing and the other part hidden inside.
Another idea is to put it on after delivery.
I just made a tea cup cake. I used gumpaste to make the handle and added 2 toothpicks at a downward angle to help the handle sit securely inside the cake. HTH.