Here is the situation: I'm looking to amplify 2 or 3 sound sources (presenter's mic, audience Q&A mic and laptop audio), with a portable PA such as this one. Behringer Eurolive B205D Active PA and Monitor Speaker B205D B&H

It has the requisite inputs, but only a speaker level output. I will need to send a clean feed (via wireless or XLR cable) to a camera recording the presentation.

Short of running everything through a mix board (the goal is portability), can I pad down the speaker's loop-through output with a simple in-line pad to get a clean line-level feed? Do I need a matchbox or other in-line device instead?

According to Behringer's official document:
http://www.behringer.com/assets/P0957-B205D-PID.pdf
That unit does NOT have a speaker output.

But it does have an output (on the back) that is switchable between line level and mic level.
You should be able to use that output with any camera with the appropriate adapter.

I have a couple of these I use as microphone stand mounted monitors. They're a bit lacking down the bottom for PA use, to be honest. I can confirm there is no speaker output - not at all what they're designed for. It will do the job, and actually sounds bigger than it is, but sadly - no outputs whatsoever.

Thanks for pointing out that output level switch, Richard. I missed it somehow. Should make connections easier than I expected.

...but sadly - no outputs whatsoever.
You must have a different model or something??
Behringer's manual clearly shows looping input and output on the back panel adjacent to the power mains input.