Interesting question yourself. I am not a lawyer, but: I think this would hinge on the definitions of "compelled" and "witness" in the fifth amendment. I think the courts would rule that being "a witness against himself" would require synthesizing thoughts into a statement then made under oath. I think searching thoughts would be considered a search.

It would be interesting to see how the courts would handle the fact that many thoughts are fleeting and not indicative of belief or intent.