I'm interested in experimenting with custom LEGO Powered Up peripherals. Are there connectors available that I can use to interface with the hub?
There do not appear to be any compatible connectors available currently. You can obviously splice cables from the official components, but that's expensive and makes those components unusable.
I hope that someone has a better answer, but the best that I've been able to do so far is to use a modified SATA connector. I first saw this in a video that demonstrated that the pin separation is the same between the two connectors.
Here's a standard SATA connector:
With a little Dremel work, you can expose the pins and reduce the width enough to fit the SATA connector into the Powered Up hub:
Here's the Powered Up connector for reference:
Notice that the SATA connector uses one extra pin. The pin on the right is extra and will not make contact with anything.
The first 6 pins line up with the hub just fine:
The SATA connector is unfortunately a little too thin to fit snugly:
If you can get it wedged in using some sort of shim (paper, cardboard, etc) you should be able to get a good connection.
This is a great suggestion, however; use the HDD end instead.
Just grab an old HDD (SATA) and use the flat pin connectors (not the spring ended ones suggested above). If you are careful with the side cutters you can actually recover two connectors from one HDD header. (One from the power and one from the data)