Yes. I installed a control wire in pin location 42, ran the wire through the firewall into the cab, connected it to a single throw, single pole toggle switch, and connected a control switch to a signal source. Pin 42 is vacant and filled with a plastic dummy plug like many of the unused pin locations in the TCM connector.
You are correct. The O/D lock out connects to pin 61 and is independent of the mode selection function. I also have O/D lockout which was installed by Freightliner.
The Allison TCM is mounted on the drivers side firewall at the top, just under the hood. It’s very easy to access.