Are there any internal USB headers/connectors?

Short answer: I doubt there's anything like it

Long answer: According to the BIOS, there are two USB devices, the USB-C port on the top, and Bluetooth (which weirds me out a bit, thought it would be integrated into the WiFi module). If you do an lsusb, you get something like this:

Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 003: ID 13d3:3553 IMC Networks Bluetooth Radio
Bus 003 Device 002: ID 28de:1205 Valve Software Steam Controller
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 001: ID 046d:c52b Logitech, Inc. Unifying Receiver (this one is not included)
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

As you can see, my USB 2.0 keyboard shows up on Bus 001. However, if I plug in an external SSD which properly uses 3.0, it jumps over to Bus 002, and if I plug in a 3.0 hub with built-in ethernet, and plug the 2.0 device into it, it shares those between 001 (keyboard) and 002 (hub and ethernet) -- I don't know the exact specifics of how USB is handled on Linux, but from brief experimentation it seems like 001 and 002 form a pair, and 003 and 004 form another pair, giving us two 3.0 slots, one of which is used internally. There is a very slight chance they left a debug port on it on some of the internal connectors, since we know it can handle multiple devices, but given the layout of the device it's very unlikely.

It's kind of a shame, there are so many things that could be done with it (I'd personally like to add a fingerprint reader).