This could work if the analytics engine is free and (ported to) JavaScript, but not if it's closed source. In the latter case, a trusted third party (escrow, one might call it) as OP described does seem like the way to go.
The problem is, why would end users trust the third party more than the analytics developer? Are there companies that specialize in being this third party and have amassed mutual trust of the general public (akin to a notary public) for handling data and code without leaking either?
The problem is, why would end users trust the third party more than the analytics developer? Are there companies that specialize in being this third party and have amassed mutual trust of the general public (akin to a notary public) for handling data and code without leaking either?