The only possible real issue with the statutory scheme is whether it shifts the burden to the defendant to provide evidence. The Virginia Supreme Court also addressed this in Magruder:
The defendants do, however, claim that the statutory procedure, by its terms, shifts the burden of producing evidence and requires a criminal defendant to call the forensic analyst in order to exercise his right to confront that witness. This argument is not cognizable under the Confrontation Clause. Instead, it raises due process concerns that are not properly before us in these appeals. Because the defendants did not avail themselves of the opportunity to require the presence of a particular forensic analyst at trial, they were never in the position of being forced, over their objection, to call a forensic analyst as a witness. In other words, no defendant said to the respective circuit court, "the forensic analyst is here to testify but the Commonwealth must first call the witness." Like the situation in Brooks, "the trial court never had occasion to address the proper order of proof."In other words, the statute might unconstitutionally burden shift, but only if the prosecutor screws up can a defendant preserve the issue for appeal. In order to give the defendant grounds for an objection, the prosecutor would have to get the demand, bring the witness in, and then refuse to call the witness, thus forcing the defendant to call the witness. I've never seen a prosecutor do that. If the defense has demanded that the expert witness be there, why wouldn't the prosecutor use her as a weapon against the defense? So, while the particular set of circumstances needed for the the objection to even be made could happen, it seems rather unlikely. No objection = no error preserved = no appeal of whether the statute unconstitutionally shifts the burden of proof to the defendant.