Trying to set up a serial link at low baud.
Is there any availability to create a serial link between 2 guests instead of having to use the NIC?
Looking to have it send a packet every now and again, so it will have software on the client and server that will maintain security between them.
How can I set this up in Qubes please?
Although Xen supports emulated serial connections between domU’s (virtual machines), AFAIK QubesOS does not support this feature.
You can use instead any of the ideas outlined in the Qubes documentation here (also onion link) … updated a couple of months ago. Or enhance QubesOS for all of us, by adding the serial feature. 
This indeed looks like a use case specifically for qrexec, not a serial link.