So your NixiSat very likely needs just three wires... +5VDC, GND and GPS TXD... Checked my assumptions and it turns out the GPSII doesn't need the 1pps signal. Works fine with just a GPS NMEA stream. Your NixiSat may well be the same.
Given just three wires, how about bringing TXD and GND in on the existing BNC and finding a way to add a +5 wire somewhere else on the clock? Might be possible to inconspicuously fit a well insulated wire between the bottom of the wood and the base?
I'd bet NMEA is supported... At least a few Oncore models could be configured for NMEA output and the NixiSat description references NMEA in the command list. Certainly the same or very close code base as the GPSII:
8) Coordinate Scrolling: 0 = disabled, 1 = enabled. You location coordinates are scrolled in degrees, decimal minutes and decimal seconds, in accordance with the NMEA-0183 specification.
As far as I know, the NixiSat will support NMEA. I there are a few members of this board who have upgraded their NixiSat's with a GlobalSat GPS unit similar to what you used. Also, the GPSII, NixiSat and NixiChron are all the same designs just in different packages.
I like you're idea of using the BNC and grabbing the +5V from under the PCB somewhere. That would be clean.
I have the schematic for the NixiSat, I can send it to you if that would help? Let me know and I can send it to you.
An Oncore footprint adapter would be powered by the +5 provided at the NixiSat Oncore socket... no need for any other tap into the NixiSat pcb. Think two wires from the BNC (data and GND) and a third (GPS data) routed inside from under the clock... The three wires terminate in a 1/8" stereo plug plugged into the adapter board. Better still, include an OSX jack in parallel with two lines of the 1/8" jack on the Oncore adapter. This leaves the installed BNC-OSX pigtail intact.
I'd be very interested in the NixiSat schematic... bob home at pacbell dot net. Let's take this off-line for a bit?
I've got 2 of the GPSII clocks, some of Jeff's earliest, and I'd be very interested in obtaining more information (all the gory details) about how to go about replicating this wonderful fix. Thanks!