One more data item:
I was able to test in Mint Linux 17.1. I used the standard install to add a printer, to my surprise it saw the printer on the LAN. I let it install it, it used the DNS-DS protocol (keywords: zeroconf, bonjour, avahi).
This Mint uses Cups 1.7.2 , Gutenprint 5.2.10-pre2, and I selected printer MG7100 . It didn't allow me to select cassette 1 either. It printed the test page not quite OK, missing most of the border. I then attempted to print my standard reference print from
www.colour-science.com, with 4x6 paper in the printer's cassette 1, with Mint's gthumb. When I selected that Glossy paper, and set the printer source to 'printer default' , it came out properly from cassette 1, although a bit dark.
I checked the Mint Cups backend directory, and it indeed uses dnsds. The printer tool from that Mint even showed ink levels (don't know how accurate).
Our Cups 2.0.0 doesn't have a dns-ds backend available, and I cannot find it under it various names on either Hobbes or this site. I have no idea how difficult this would be to port. The Ahavi diagram on Wikipedia didn't look simple.
Even if it were available, you'd be dependent on Cups translating the OS/2 4x6 request to the proper printer cassette. No easy fix, I think.
Regards, Mat