A few years ago we bought a 727-200 with no long range nav, later LRNs were installed. In the period of our operating this aircraft before the LRNS were installed one of the other pilots brought his Marine style (for boats) GPS off of his boat, proped it up on the windscreen and we used it with no problems.
Now, we never, repeat never told ATC that we had LRN style equiptment on board nor did we ever (at least I didn't) use the boat GPS for anything but judging progress on long legs.
It was amusing however to read the upper winds as current. "Current 320 degrees at 110 knots."
