Okay ... with a voltmeter, find out the voltage the optical receiver puts out while a signal is being passed. Take a SPDIF and pass signal through that and measure the output voltage as well. Use the SPECIFIC digital gear you'd be using in the field. If they are similar volatages, then you might be able to do a straight swap, keeping your signal and ground the same, and just not connecting the analog to anything.
If they do not output the same voltage, you would need to construct a buffer circuit similar to those in an external box, I'd assume. If you're good with soldering, it shouldn't hurt to try the swap anyway b/c you can always solder the original piece back on if it doesn't work with the coax.
*disclaimer* this is your gear - if you slip with soldering or feed a nonstandard voltage into the unit, damage can occur. Not to discourage, but just to let you know that bad can happen.
Let me know how it goes and if you need any help. If you can post a close-up of the jack, I might be able to tell you which is signal and ground if you can't. You can probably get an RCA jack at radioschwag.
-Matt