[dsp1234]

Under extraordinary circumstances, compelled expert witness testimony has lots of precedents,

Can you name some of them?

IANAL, but the Federal rules of evidence section 706 explicitly states, "But the court may only appoint someone who consents to act."

[legalbeagle]

Compelling expert testimony happens all the time. (You might have to pay for it, but the witness has no choice.) The Apple subpoena is different. It is an attempt to compel engineering and coding work.