Not it, too. I’m in the data-sending business, not receiving. Once the packets are up, who cares where they come down?
Not it. 
This is why I stick to my “ghetto LED” tester … It’s easy.
All this talk about catcher had me thinking. Rather than dealing with the timing issues, couldn’t one take a ws2811 chip and monitor the RGB pwm outputs? Might be a nice way for a controller to know its signals are getting to end of the bus. 
@Robert_Atkins interested in your results here as I will be building a similar 30x20 array soon.
That’s what the ghetto LED is doing, essentially. If the signal gets to the end of the bus, the LED will be flickering.
@Brian_Dowling Now I kinda sorta know what I’m doing, I’d really love to build a “Version 2”; I’ll do a write-up soon and coalesce the various pieces of good advice I was given here.