I finally turned the soldering iron on ;) and got some work done!
I've pocked around the ODB adapter and managed to get an NPN transistor with the collector connected through a resistor to the 5V rail and connected the UART TX line to the base of it. Then connected the collector to the base of the NPN that drives the K-Line.
This way I don't need to have the interrupt inverting "thing" that could ruin the UART timings.
Need to take a photo to post... :)