Not likely to work: "We just want to search your house for 30 minutes" or "you can have your document back after we photocopy it, so no warrant needed."
The idea is that they cannot do certain things without a warrant.
I can't exactly go re-write iOS or Android (technically, I suppose I could do the latter) in order to add this feature.
This scenario would be quite a bit more akin to "let me have that locked briefcase you're carrying through security, no need for you to unlock it" and then returning it to you locked after 10 minutes.
Was that a 4th amendment search if they claim that they didn't look through your briefcase?
It's a seizure whether or not they looked through it, and it's a search if they examined—by any means, technical or otherwise—the contents in a way they wouldn't have been practically able to without seizing it, irrespective of whether they opened it (and even more independent of whether they claim they opened it.)
If it's optional and I can and will refuse, fine. If it's not,and I'm not attempting to cross a border, they can get stuffed.
For domestic travel, they do not need to examine the data - only the physical device needs to be examined to rule out security risk. They can do that non-intrusively with imaging and chemical analysis technology.
The idea that someone could claim that a limited intrusion into the contents of my communications isn't an intrusion at all is absurd.
Is there any niddling contract law in purchasing plane tickets?
Chief Justice would probably be all: "LOL, you did wut? ಠ_ಠ"
I wonder how long it would take to clone a phone and give back a ringer with some extra software installed. It's not like most people know the serial number of their phone. And if the phone's in a case, then put the ringer in the target phones case, and it has the same look and feel.
Especially if I've got a semi-obscure phone in an unpopular color. That's at the level of a targeted attack against an individual.
However, from a security perspective it would be awful if they could disappear with your device whenever they wanted - that certainly makes me uncomfortable.