I've got a large icing tip (larger than the Wilton 1M) and I don't have a heat core, so was wondering if the large tip would work in place of a heat core. Any thoughts?

Don't see why not. It would be kind of like a mini angelfood pan core.

Flower nails work great. Put them upside down in the pan and fill batter around them. I do it all the time.

Let us know how it works.

No harm in trying but I don't think it will help, you need the heat core to be long enough to stick out of the cake so the metal sticking out can conduct the heat into the cake. The 1M tip is so short that it will quickly be covered by cake and no longer exposed to the air.