I probed around and found the second serial interface on the Buffalo WHR-G54S router:
This is the RJP1 header that already provides the first serial interface. Next to it there are some resistors.
This group is connected to the processor with 4 traces. The read marker (1) on the image shows where I scratched and probed them.
The second serial interface is available on the right hand pad of the unpopulated R48 (RX1) and the right hand pad of R46 (TX1). I soldered some wires (see image) and now I have both of them on the connector.