Adapting what I saw on the diagrams and what I know about the cooling system on my 92 300SE (W140 chassis), the circuit you interrupted by plugging #5 does two things:

1) Provides heated water to the windshield washer reservoir - that's the circuit with hoses #107. Note - this isn't windshield washer fluid - that's a separate deal. This is for the heat exchanger coil inside the washer tank to prevent freezing of the fluid.

2) I think from there, #14 feeds the heater cores, with the outlet of the cores going to hoses #29 and from there to the duo-valve assembly.

If you're going to the shop with it - I'd just leave the cap on the overflow tank loose to prevent pressure from building in the system.

So - if you leave the heat off, and plug #5, you should be OK for a short trip. I don't know if the duo-valve assembly is capable of holding back pressure from the cooling system - they're probably only rated to control flow.

Likewise, there's a thermostatic valve on the heat exchanger in the washer reservoir, so the coolant shouldn't back flow too much if you leave the cap off.

This is just a temporary solution.

However, if you plug hoses 14 and 107, then you're good to go for quite some time. For that matter, since you live in Texas, you're unlikely to ever need the washer fluid heater, so you can probably just use a connector barb between 14 and 5, and eliminate the circuit to the washer tank by capping the port at the circulating pump where hose #62 attaches.

Hope this helps.