On the AT3 module I noticed that sensRcore can enable the LED to signal Tx events (0X68), and I was wondering if there was a way to push that signal out to an OI pin.
No, it is not possible. The EnableLED (0x68) command can only be used to turn on the LED in the ANT module. It is not possible to use SensRcore to turn on an IO on every Tx event either.
You can use an ANT channel input mask (0x90) to map IOs to incoming (Rx) messages.