It's hard to say, but it's not impossible. Since the Homolka / Bernardo video debacle-- where their first lawyer failed to disclose a video of the couples' various crimes-- many law societies "clarified" the ethical obligations of criminal defense lawyers: that such evidence ought to be disclosed, but in a way that also protects the lawyer's client.
One of the ways in which a lawyer can meet his or her obligations-- that is, both the duty to his or her client and the need to disclose evidence to the police-- is to do so anonymously. Thus, the police get the relevant evidence, and there is no obvious link back to the client (the way there would be if the lawyer himself disclosed the evidence).