I now understand why I couldn't access or control the HPS_LTC_GPIO signal on LTC pin 14 - vendor buffoonery

FOUND A SIMILAR COMPLAINT SEARCHING THE WEB

I came across another hobbyist who had the same experience as me - other HPS GPIO signals could be read using the GPIO memory mapped interface, reflecting whether the external pin associated with that signal had 3.3V or 0 applied to it. However, they too were unable to read any value or set it to output a 1 state. This was for a slightly different board, the DE0-Nano but it was a clue. 

In the helpful response, somebody mentioned that the schematic shows a DNI (do not install) for the resistor that connects the HPS chip pad for HPS_LTC_GPIO with pin 14 on the LTC connector. In other words, pin 14 isn't wired to anything. I opened the schematic for the DE10-Nano board to see how that pin is wired on my board

LOOKED AT ACTUAL SCHEMATIC FOR THE LTC CONNECTOR OF DE10-NANO

Darn - same DNI notation illustrating that we have no connection at all to the HPS chip pads. 

COMPARING THIS TO THE MISLEADING USER MANUAL FOR THE BOARD

If instead you read all the user documentation this GPIO signal on LTC is listed as an available resource. Here is the schematic and table from that manual which misleadingly show that signal connected to pin 14. 

You can see that the table clearly implies that the pin 14 signal is connected to Cyclone V chip pad H13, yet the schematic and my tests prove otherwise. The diagram above the table shows a direct wire between pad H13 and LTC pin 14, wherease the schematic shows a resistor position in that line with an infinite (missing) resistor. 

VENDOR BUFFOONERY STRIKES AGAIN

The Terasic boards such as the DE10-Nano were the official reference boards for the Altera Cyclone V system on a chip, with the golden reference design based on those boards. Intel bought Altera's business and continued to publish the golden reference designs, documents and everything else, but obviously nobody has actually looked closely enough to see that this could NEVER work and has not worked on several versions of the development boards. 

That accounts for a few hours of my time wasted entirely, some of it attempting access to a pin that is not connected to anything and some of it spent with documentation poring over the many thousands of pages.