According to Ted Langdell on the SFCutter.org list:
You can open the shell of most Mini-DV cassettes with a small jeweler's phillips screwdriver. If the tape is not broken (doesn't need splicing) you might be able to straighten out the problem yourself and see how it plays... then capture either side of any video that FCP won't capture.
I'd do this on a table with a clean, light colored floor below, and a large white piece or pieces of paper on the work surface to easily see where any parts roll, fall or fly. Be careful to note where the spring-loaded parts go so you can put them back together. If you can videotape the take-apart, that may be helpful in re-assembling the cassette.
It the tape does need splicing, (perhaps because it's stretched into a string) the tape width is close enough to 1/4" that you can likely use a 1/4" audio splicing block and tape to make the splice... then transfer or capture.
If you need the timecode intact, cloning the tape via firewire to another deck or camcorder is the way to preserve the original time code. Otherwise, when captured, just output the captured file to another tape for preservation. That may cause some issues when/if you have to batch recapture, though.