I kind of got the Denafrips Iris working, although not 100% yet. I had to change the piCorePlayer Squeezelite Output setting. It's connected to the same USB port, but apparently it sees the Iris as a different device on the USB bus. (Makes sense in retrospect.)
However, the HDMI cable pushing I²S to the Pontus seems to create lots of static. (Tested with all of the I²S input wiring modes to no avail.) I eventually switched to S/PDIF Coax, just to see if it would work, and that sounded much better. I've ordered another slightly more expensive HDMI cable. The first one was $11 and the next is $25. (There are people out there paying $300+ for a cable to connect a $1000 box, but apparently any good cable will work fine.)
After switching to Coax, I still heard little clicks and pips, especially during the silent period between tracks. And the general sound seemed irritating. I had a hunch it was timing problems (dropped samples, etc.), making me question why the Iris permits input speeds higher than the output speeds. USB input can be up to 768KHz, but on the output I²S maxes out at 386KHz and the S/PDIF can be only 192KHz.
As a test, I told Squeezelite to upsample only to 192KHz, matching the S/PDIF maximum output. Now all the clicks are gone, the the grating quality of the sound has disappeared.
Of course, after all of the futzing and noise, I couldn't possibly tell you whether it sounds better than it did before. Once I get the new HDMI cable, I'll test it again, and try to rig up some type of A/B. Perhaps I'll bring my Pi3 from the Family Room, and feed it directly into the Pontus' USB input. And now I have to order the firmware upgrade for the Pontus II.
Oh yeah, and I've ordered my firmware upgrade.