Well dang. I flashed the XtreStoLite G900T 1.3.5a ROM and the faux123-006u kernel (from here)
and it got mirroring to work w/ the Chromecast...but its doing the thing where it only shows the top left corner of the screen all zoomed in. Same thing with the kernel XtreStoLite included.
Once I get an actual "mirroring device/dongle" I'll try that with the faux123-006u kernel.
Kinda hilarious that the Netgear Push2TV is STILL cheaper than the official pioneer usb/MHL/HDMI cable which, fun fact, didn't work with w/ my S5.