Virtualbox has multiple networking mode that you can choose for guest VMs. You did not specify what mode you chose, but if you chose 'NAT', (it's the default and most popular), you cannot ssh from the guest to the host. Because there is no route from the guest to the host.
Additionally, your guest OS does not know it is a VM. When you guestuser@guest$ ssh -p 2222 hostuser@localhost
you are saying: ssh into localhost, the guest OS, port 2222, and connect to the sshd service running on the guest OS.
If you wish to have the guest OS be able to connect to the host OS you should change the VB networking mode. You will probably want 'Bridged' mode. Here the VM acts as a separate (physical) computer on your home network. It will receive a IP address via DHCP. Then you may ssh from one into the other, assuming there is no firewall rules or other 'barriers' between them. There is no port forwarding in this mode; all ssh connections will be through port 22 by default.
Example
hostuser@host$ ssh guestuser@192.168.0.2
guestuse@guest$ ssh hostuser@192.168.0.1
Assuming host IP=192.168.0.1 and guest IP=192.168.0.2
Opinion: Unless you are setting up a specific service on your guest, there is often little need for the guest OS to be able to connect its host in the way you are asking about. I would not go through the trouble of setting this up.
Do you think this is a good question for superuser.com or would I stand a better chance of getting a good answer if I moved it elsewhere? – PSkocik – 2018-08-18T12:29:06.027
You should always elaborate on "not working yet". There is no reason why it shouldn't work, unless you have firewall rules that prevent it. – RalfFriedl – 2018-08-18T12:51:51.317
@RalfFriedl I set up port mapping in Virtual box, from Host/2222 to Guest/22 and from Guest/2222 to Host/22 and unblocked incoming 22 and outgoing 2222 in the Windows firewall, but only incoming host-to-guest connections work. Can't call out. – PSkocik – 2018-08-18T13:09:40.007
@RalfFriedl I get
connection refused
even if I turn the firewall off completely. It's not a firewall problem, I guess I just need to get Win7 to somehow forward to the the Host at TPC/22 if I connect to 2222 in WIn7. – PSkocik – 2018-08-18T13:13:37.350Why do you try to connect to localhost instead of the host you want to connect to? – RalfFriedl – 2018-08-18T13:15:28.893
@RalfFriedl So I could rsync from inside and bundle it with my build command. I could avoid the rsyncing part completely if I got the shared directories working right, but https://superuser.com/questions/1350225/virtual-box-shared-directories-not-showing-up-in-cygwin-when-sshing-into-it but I'd like to be able to do this just for the sake of being able to do it too.
– PSkocik – 2018-08-18T13:33:56.217