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Could it be argued that the knowledge of which finger (or part of your finger) you use for authentication is equivalent to a passcode?

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Maybe, but you get five tries before you get locked out, so even if your lawyer pleaded successfully with that, the judge could reasonably order that you try with, say, all the fingers of your right hand, or maybe both thumbs and index fingers, without forcing you to disclose that knowledge.

Conversely, is there anything preventing them from having you try all ten fingers?

Yes, you are locked out after 5 incorrect prints. At that point, you need to use your passcode to unlock your phone.

That said, even if they guess that you're most likely to use your thumb, there is still a chance you can't unlock the device, especially on pre-6S devices, since the sensor is a lot more finnicky.

Chance. It'll stop accepting them after 5 attempts according to other posters in this thread. Combine that with the fact you don't know if it read properly or not you really don't have good odds that you'll succeed.

Plus, I think you can use your knuckles too.

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