figured it out (maybe)
A little late coming back, but thanks for the pointers. I dug into this a bit more on the transmitter/receiver side. Circuit details if you're interested: It appears the transmitter, utilizing a common IR LED, is driven by a transistor driver on the low side (NPN) which is driven by the output of an oscillator involving the LM2903 comparator. The oscillator is made so that the output consists of about 20 short pulses of 5 ms each that occur every 100 ms or so. I didn't read the exact timing. See attached diagram.
The receiver utilizes the Siemens part SFH 506 which is a IR receiver with signal conditioning circuitry built in to remove dc, apply AGC, etc.... See the TSOP17 from Vishay - a cross. It's output follows the pulses, but removes the short pulses. That drives a transistor driver that acc. to the circuit, if powered with 24 directly (as most garage openers do) would sink 750 ma. Plus no evidence of the data would appear at the opener unit, so I concluded the source contained a series resistance of about 60-100 ohms. When the driver was activated by the receiver IC, the supply voltage is dragged down, producing data at the opener unit.
I also noticed the "gnd" side of the receiver IC is floated, clamped to the supply voltage by a 5 V zener, so the receiver IC is supplied with 5V no matter the input voltage.
By the way, the driver transistor on the transmitter was blown. I though for sure it was the Siemens part. Happy to find it was a lot simpler part to find.
thanks for readin',
Shawn