I think you're all looking at it the wrong way. No need for SCP, FTP anonymous is enough. Just configure your ftpd to only support anonymous writing, and no deletion nor reading (or move have a crontab move the files in the directory to a more secure location, inaccessible from FTP). Give the user a quote of say 10MB per day max.

The problem is that a lot of shared wireless connections don't allow SSH or FTP access. In that case, i would recommend using wget to post the data to a webpage. This is even better, as it allows to view the data for authenticated users, but not the guy who stole your device.

Even if you go with the SCP route, this isn't a security issue. You can create a dedicated user account who can only go to /home/spy and doesn't have execution rights to anything, and also has disk quotas, etc. Even better, set the shell to /sbin/nologin.