I'm sorry that I had to step away from my computer and have just now seen your responses to my questions.
Most lumps and bumps on dogs aren't terribly itchy so it's interesting that he scratches the one he has. They usually bother us more than they bother the dog.
If they're located over pressure points such as on a leg, for example, then there may be nerve compression or tingling to which a dog could respond with licking.
If Pippin's mass is located over a rib, then this could be why he licks at it.
However, if it's located over his abdomen, then it would be less clear why it would bother him.
I've seen a few dogs lick small masses out of habit. Something triggered their initial interest in it and they simply continue to mess with it. I don't typically consider licking of a mass to have a negative connotation, though; it doesn't mean that this is more likely to be cancer if this is your concern.
I can understand your reluctance to remove the mass, under the circumstances, since he has some heart issues; however, if it's small enough, it could be frozen off without the need for anesthesia. Not every vet has the equipment to do this procedure but it might be worth calling around to see if someone in your area does.
If you haven't tried any topical products to discourage him from licking the area, then this might be another avenue to take. Bitter Apple can be spayed on the lump which might deter him since it has such a nasty taste. Or covering it with a very small amount of jalapeno juice may also have the same affect.
Personally, I wouldn't be too concerned about his licking this bump but I do know that I would find it somewhat annoying. I wouldn't go so far as to recommend that he wear a cone but perhaps a t-shirt might prevent access to the area and he'd leave it alone.
I hope this helps. Deb