"For a dayhike up Whitney from the JMT, I do not think any one will see whether you have a cannister or not since most day hikers dump their packs beside the trail."
Lots of backpackers will dump their packs at the junction 1.9 miles down from the Whitney summit. They hit the summit and return to their packs, and then continue the backpack route. Interestingly, the local marmots have learned this, so they work as a team. They run around the backpacks enough to sniff out where the unprotected food is, and then they chew through the pack. This makes a marmot-size hole in the pack and allows them access to the pic-a-nic basket. The marmot team takes turns feeding there until they are full or until the owner returns and finds his $200 backpack half-ruined.
Approved canisters are not required for day hikers who are not camping within the Whitney Zone. They are required for backpackers carrying "overnight food."
Been there. Done that. Read the book. Saw the movie.
Besides, lots of ultrarunners and fastpackers don't really camp. I've seen some who curl up in a ball for a few hours of sleep, and then continue.