So while it would appear that the MLS has five Concacaf Champions League spots locked down between the USA and Canada, it is possible that only three would qualify. Both the US Open and the Canadian Championship could theoretically be won by teams outside of the MLS.
In that event there would only be three MLS teams in the field. In theory, the three would be: 1) the MLS Cup Champion, 2) the Supporters Shield Champion, and 3) the Champion of the MLS Conference opposite the Supporters Shield Champion.
However, since Canada's lone bid goes to their Tournament Champion regardless of their MLS accomplishments, there does exist an unlikely scenario where the MLS Cup Champion, the Supporters Shield Champion, and the Champion of the MLS Conference opposite the Supporters Shield Champion could all be left out of the Concacaf Champions League.
-A Canadian Team would have to win each MLS Conference. Let's say Vancouver(West) and Toronto(East).
-One of them wins the Supporters Shield, while the other wins the MLS Cup.
-A non-MLS team (or Montreal) wins the Canadian Championship.
In that scenario, the four spots allotted to the USA would go to the US Open Champion, as well as the top three remaining US-based MLS teams from that season's standings.
The MLS Cup Champion and the two MLS Conference Champions (including the one that won the Supporters Shield) would all be sitting at home watching from their couch.