It appears that the I fare class on a CO codeshare for a CM flight is Business class reward redemption. I can't find the description of that fare class on co.com, but trying to book a reward redemption IAH-LIM (including via PTY) in BF/F Saverpass seems to pull up this "I" fare class. The same "I" class on the IAH-PTY CO portion does elicit Intl Business Class and appropriate seating. Yet for the CM portion it consistently displays a notice like:

"There are no First/BusinessFirst/International Business Class seats available on CO9057 in the Lima, Peru (LIM) to Panama City, Panama (PTY) market."

Even a simple booking for Business Class SaverPass (20k miles E class) PTY-LIM shows availability for Business class on the Embraer 190s, but then the website gives inconsistent messages such as the above when I get further along in the booking process.

It seems that there is something wrong with the CM interface on Business Class reward travel.