I'm thinking I can use a plug for (among other things) a Network Time Protocol (NTP) stratum-1 server.
To do this, I'll use a Garmin 18 LVC/OEM GPS connected to the second serial port as the stratum-0 device. "Second serial port?", you ask. Yes, the one that can be routed out the SDIO pins. I should be able to hack together a cable from the SDIO port and change the kernel to use two pins as the second serial port (TX/RX) and a GPIO as an input (for the pulse per second (PPS) signal). Then, I should "only" need to put together a bread board with a MAX3232 (plus capacitors), a USB connector (for 5V power), a connector to the SDIO port cable and a connector to the GPS cable.
The GPS and 5V side of the MAX3232 will be powered by the USB connection (I'll use a powered hub) and the TX/RX/PPS signals will level shift through the MAX part.
Considering my day job is writing SOFTWARE, does anyone see a problem with this logic?
By the way, this is not a new idea. There are write-ups for doing this on other little linux computers elsewhere on the net. The key, I think, is a second REAL (not USB) serial port and the PPS signal.
Please don't ask why I want to do this. I have a wife that will be happy to ask that question.