So the code will live on github, see the link on the project main page.

There are 3 things as of now:

1. i2c bus scanner, to see if the si5351 can be found
2. a small frequency setting sketch, to see if the library works
3. A small sketch (si5351_antenna_analyser.ino) that accepts a new frequency over serial, with an accompanying python script that sweeps between 27 and 29 MHz.

The wiring is shown in above photo, which is really straightforward. The red/brown pair is power, the yellow/orange pair is i2c, and the green one is connected to clk1 to act as a rudimentary antenna. No passives required anywhere.

So the setup is working now, I can see a nice frequency sweep around 28MHz in GQRX. Next up is the resistive SWR bridge, and the necessary adjustments to the main sketch to push the resulting swr value over serial to the python script.