Not exactly something I'd recommend if you want to use it regularly, but the clip-on pickups sold to plug into your tuner actually do a fair job of picking up the instrument - if plugged into an amp instead of the tuner, of course.

Performance is so exceptional that at one jam I had the clip on the pick guard of my mandolin all night, but didn't bother plugging into the amp; and four different people congratulated me on how much easier it had been to hear my licks that night.

John