[elwebmaster]

I don’t get what you mean:

Machine A -> public IP Machine B -> behind NAT

You can always SSH from Machine B to Machine A and expose any local Machine B port to the internet as a port on Machine A. The feature is called GatewayPorts and is a standard SSH feature (at least in OpenSSH). It does not require installing anything other than SSH on Machine A (or even Machine B, unless you want a script to re-establish the tunnel upon disconnect).

[LargoLasskhyfv]

Anywhere, Anyplace, Anytime -> public IP running that thing and pointing to -> that thing running behind your dynamic and NATed IP -> golden/Bob's your uncle/gotcha covered/whatever