Native development log / API access?
Craft has Craft::log() for that purpose. Brandon has an example of this function over here

To make this work from an external request, you can create plugin with a controller which saves incoming data (post or json) to the log.

To secure this with a key just send an additional variable and check if it's correct.
Take a look at Ben's Dump plugin, for an example of such a key-protected controller.