I would also like to make this pareve and substitute coconut milk for the heavy cream. For those that have already made this substitution, did you...

No I have not made this one, but I have converted other recipes from dairy to parve.
I find it helpful sometimes to borrow a recipe that was built parve or vegan from scratch, rather than tinkering with an existing one. That way someone else has already done the testing and adjusting.
Here, for example, is a non-dairy caramel sauce...
http://www.onegreenplanet.org/vegan-food/how-to-make-caramel-sauce-without-dairy/
Hope this helps.
Happy new year!

Funny, I'm making it today, too, but I don't have to make it parve. I think I'd probably use margarine, but can you test it first? I have no idea if margarine and coconut milk will make a decent caramel, but you could try it. Worst case, you could just dust the delicious cake with powdered sugar if the caramel doesn't work.