I am using a Rocktron 5-7pin midi cable to send power to a Ground Control Pro. Is there a way use the (now empty) 9V INPUT as a 9v OUTPUT to power other standalone 9V pedals on the pedal board by "daisy chaining"?
Interesting question. I suspect the 9-V Input and the MIDI pins supplying power to the GCPro are connected internally, so it should work in theory. However, I honestly cannot say for sure, so I'll defer to John Clark for the definitive answer. In the event they are, however, the 9-volt number isn't as important as the current draw of the pedals in question.
When daisy chaining multiple pedals, the power supply involved usually delivers 800ma-1000ma of current. What one needs to do is total up the individual current requirements of each pedal to ensure the power supply being used can deliver what's needed, so you would have to make sure whatever is supplying power to your GCPro can provide enough current for the additional pedals.
Here's a web page from Beavis Audio Research
with a lot of good info on powering pedals.
Hope this helps...