Run a Tunnel on existing Comcast cable?
This one may be a bit tough. If I understand correctly, you really want to use the same coax. from pole to house for both incoming signal to the Comcast access point and a local network to the barn. I can't say for sure that it won't work, but I really, really wouldn't count on it. You might be able to get MoCA working, but I'm not so sure. Keep in mind that you would be throwing your own &quot;local&quot; network on top of what is (past the Comcast box) Comcast's network.
My recommendation would be to run another wire, or possibly fiber, from the house to the pole to the barn. Keep in mind that 500' is way over normal copper Ethernet distance, so this would not simply be ordinary 4-pair CAT 5e or 6. You could go with a line driver of some sort (there are types for twisted pair - even old CAT 3 or worse, as well as for coax.), but this might be a great one for fiber. Fiber has some big advantages:

Speed
Distance
Immune to electrical interference
Non-metallic

The converters needed have gotten relatively inexpensive. Just an example, no experience with this particular brand:

For the fiber cable, unless the coax is in conduit, you will want to look for outdoor-rated cable, possibly armored. Make sure it is the right type (LC, SC, etc.) and I would seriously consider getting pre-terminated cable rather than doing it yourself for a one-time fiber install.