I have my home-office in a basement guestroom. This is also the room with the central electrical panel and where the cable-TV and satellite wiring come together.
The cable modem that I use for primary internet access is located in that room and hardwired to it a firewall that I use to create a satellite to the main-office network. Now I need to create a wired connection from the 'safe' side of that firewall to two other rooms in the house.
First question: Physical Layer
When you say "come together" are there individual "home-runs" to every co-ax jack in the house that all come back to a junction panel in that room, or is the house a series of one-room-after-the-other T-connections?
Re-using the co-ax cable is maybe possible if they're all individual circuits and you don't have to share the TV signals to places where you also need Networking. Multiplexing the TV signals and the Ethernet (or other protocol) is trickier. Are there already unused co-ax jacks where you need to go?
Wifi is out and I am not so thrilled about using an ethernet-over-powerline adaptor as the one I have is rather slow and not terribly robust (needs occasional re-setting to maintain its speed).
I've never had great luck with those either. Some people do. Mine are sitting in a box in the basement un-used after trying them for a while. (They also make a hell of a racket on some Ham Radio HF bands and I didn't really want to add to that bigger picture problem. "Broadband over Power Lines" being mentioned in most Ham Radio groups will get you shot.)
The design of the house doesn't lend itself to the stringing of cat5 without having to drill through concrete and rafters. So if I could make use of some of the otherwise idle coax runs to carry network traffic it would really make my life easier.
How about access to return air ducts? Ethernet is low-voltage and in almost all cases, it can be run INSIDE return air ducts. I recommend that to people in older houses.
It works well to get from floor to floor, and you can "pop out" to closets, etc... usually it's fairly easy to find a way to get from "here" to "there" via return air ducts. Plenum-rated cable, and if you're a geek/picky about things, a little fire-stop putty to seal up the ductwork wherever you pop in and out of it.
Don't use forced air ducts though, I believe in most places that even having low-voltage electrical running through them is a no-no, per the electrical code.
I'm no electrician, but I've seen both done. If the forced air ductwork gets hot enough to melt Plenum-rated Ethernet cable, that's definitely not a good thing.
One common method I've also seen is to go up a return air duct, as high in the structure as you can, then pop out the top and continue up the wall into an attic above the living space, after drilling a hole in the header at the top. Then pop back down into closets or into upper story walls from above.
Does anyone have experience with consumer grade Coax MoCA adaptors (or any other method to feed ethernet via coax) ?
What brands to use ?
Haven't needed to use them.
Any security issues to be aware of (is there a way to create 'pairs' of transmitters that only talk to each other) ?
Typically no need.
Twisted-pair cable is intentionally "leaky" and signals in theory could be read inductively by just putting a device around the cable itself. If someone has that kind of access to your house, there's easier ways to get your data.
Co-ax is designed to keep signals inside it. The outer conductor is called a "shield" for a reason.
Can't really recommend a product for the link layer conversion, but I haven't run into a house yet where UTP runs couldn't be made... just have to be a little "creative". And yeah, it's a bunch of work.
There are companies who'll do this type of thing... they'll come in, you tell 'em where to put the main patch panel, and they'll put jacks in every room with four or so RJ45 connectors, or a combination of RJ45 and co-ax connectors fed with 75-ohm (TV) co-ax. They figure out where the holes need to go, and patch things up pretty nicely so you don't see the mess.
There's one run of both Ethernet and co-ax from my basement to the living room. The former owner found a spot between joists from the basement and drilled through the overhead drywall, right up through the floor in a corner of the living room. He put up some little square conduit on the ceiling along a ridge where heating ductwork lowers the ceiling to hold the cables, and it looks fine for a basement. Can hardly tell it's there.
In the living room, it comes out behind the entertainment center, so no one sees the small hole in the hardwood oak floor.
Originally it was just co-ax... and since there was some extra, I just taped some Cat 5 or 6 (whatever I had) to the co-ax and used it as a pull cable to pull up the Ethernet and then crimped on the RJ45 once it was up in the living room.