Is a wireguard setup possible between two openwrt devices, call them A and B, that makes peer A appear as though its traffic originates from peer B but that uses the connection bandwidth of itself (peer A)?
My peer A is on a fiber connection and peer B is on a slower cable connection. I want peer B's traffic to appear as though it is behind peer A's network, but use the actual bandwith of itself (peer A).
Is this possible?
If A and B are both on internet, B will generate own traffic, while A will carry it's own traffic plus the traffic of B.
B's traffic will appear as comping from A, but I don't know what good it's going to do, except for getting around geo blocking.
Is it possible to have A not carry both A+B's traffic somehow yet allow B to appear as coming from A?
Not really.
What are you trying to achieve?
Use case is geo blocking but peer B has a slow connection. I would like peer B to have peer A's IP WAN address but use its own bandwidth so it does not saturate peer A's bandwidth.
You can cap B at A, but B is obviously never going to be faster than the plan bought from the ISP.
You're trying to ask netflix "please give me that stream" via VPN, which is a tiny request, but make netflix respond a couple of GBytes of data without going through that VPN, I guess.
What you're talking about would result in "asymmetric routing". Make the request go one way, but the response should go another.
While this is technically possible "in general", I's also most likely blocked by your ISP.
You might read up on amplification attacks as a sub category of denial of service attacks. Find a tiny request the causes a huge response package, find a server that responds to that kind of requests and fake the origin address so that the server you're asking sends the response to your victim. There should (and will) be something in place to prevent this.
If you're a data center provider or own some fibers from Europe to the USA, asymmetric networking is totally a thing to do - within the network you own. But it's increasingly unlikely to be allowed/supported by your network peers if you're crossing line ownership boundaries, and it's close to impossible for contractually domestic internet connections.
Even if it was supported by your ISP and all network providers in between:
Using a mangled source address where netflix thought the request came from your home address instead from your VPN endpoint would cause netflix to apply its geo-blocking rule, since netflix doesn't want you to serve this data if it came from your home location. The very act of adjusting the package source address of your request would completely undo your attempt of circumventing geo-blocking.