I love this idea!
How about a ‘recover the stolen plans’ scenario? I don’t recommend planting anything in a public location these days, but instead of retrieving something physical, recovering the plans could consist of a close-up photo (via cell phone, for example) of an object?
For example, you could deliver a message to your ‘agent’:
Greetings, Agent 002:
Important documents have been stolen from the Embassy. Security was able to locate and remove the responsible party, but the documents were not in his possession. We suspect they’ve been hidden at <local business>.
Your mission is to recover these documents! You have <x minutes/hours> to do this task before the container they are in self-destructs and the documents are lost.
Warning: counter-espionage activity is on the rise. Be on the lookout for potential enemy agents. You must not be compromised.
Attached is a photo of the container to aid you in your search.
Send them off with a picture of, say, a specific mailbox in a crowded public location. They have to correctly locate the container based solely on the information you’ve provided and get a close-up photo of it within the timeframe to succeed.
Re: discouraging face-to-face contact in an interception scenario, I’m not sure how you could do it. Part of LARPing is face-to-face contact. I would think the best method to handle it would be to allow F2F contact, but have a ‘referee’ on-site whenever that sort of mission was being run who could act as a GM, allowing the agents to resolve ‘combat’.
For example, Agent A is tasked to deliver Secret Plans™ to Agent B. Agent B is standing by the fountain in the mall. Both agents need to wear a red tie so they can recognize each other.
Agent C is tasked to intercept the package, and knows where the meet is to happen and the recognition sign. Obviously, he cannot challenge both A and B at the same time, so he plans to either intercept A before he gets there, or tackle B after the handoff.
This sets up the scenario, and gives all three of them a chance to affect the outcome.
Suppose C decides to intercept A before he arrives. He steps in front of A as A is walking through the doors. At this point, he can try to bluff (“Our meeting point was compromised, so I came here”), or he can engage in ‘combat’. If the bluff works, no harm, no foul, except agent A loses points for being dumb.
If ‘combat’ is needed, one or both agents call the referee on their cell phones. The ref is already stationed nearby just in case. He shows up and they resolve combat however the rules allow for.
That’s just one method. The problem is, I really see no way to handle a situation that requires F2F contact without actual contact. And allowing the players to resolve it themselves is just asking for trouble; you need oversight.