Let's say someone is setting in a coffee shop wireless connected to the internet and they send an HTTP request to johnsveryownserver.com. Server-side, is there any way that I can determine the MAC address of the wireless access point that they are connected to? (Note that I am not interested in the MAC address of their machine.)
If I can't do this with a plain old HTTP request, is there anything I can do in-browser (e.g. via javascript) in order to scan for the MAC addresses of any nearby wireless access points? This might even be better because I can collect all nearby wireless MAC addresses.
I'm trying to avoid having the user download a plugin or an independent executable.
No, you can't. Using a plugin is your only option (it may be possible with Java, but that's outside my area of expertise).
I'll say this might be possible because there are people far smarter than me.
However, it is hard enough to get the local MAC address by querying WMI using client side scripting, let alone attempt to query past the local machine. I'm confident in saying that this is not currently possible to accomplish, and if you do, you'll be missing out on a large set of data due to browser security settings halting your script.