This is not really a plea bargaining issue. That is a court supervised process and it would be highly unusual for a court to approve an arrangement like that. The police accomplish this sort of coercion by promising the informant not to refer the case for prosecution in the first place.

Unfortunately there is nothing illegal about the practice. It's not extortion to coerce someone with something you're legally entitled to do. We'd need a new law banning the use of criminal informants in situations like these. (Ideally, banning the use of criminal informants without a court order.)