I'm not how viable this would be. There are hundreds of "legal highs" out there. To fully understand their safety you'd have to run clinical trials with hundreds of subjects for each drug. Not exactly a cheap proposition.
A good primer on why we need testing is the Bunk Police's documentary "What's in My Baggie". In it they travel to festivals testing people's drugs. Nearly every drug they test is misrepresented. Most people believe they have MDMA when they have "bath salts". Even the positive tests may be adulterated or one of several compounds that give similar test results. www.youtube.com/watch?v=dYzmZ1IU4zY
Not that MDMA is safe; it's without question a dangerous drug. Hundreds of thousands[1] of people taking unknown substances in unknown quantities is a much much larger problem.
[1]Electric Daisy Carnival alone attracts 150,000 people and runs 3 nights. There are several other festivals of similar size, and dozens that attract tens of thousands per night.
A great example of what can happen is Fen-Phen, the diet drug that was taken off the market a while back. The drug fenfluramine also activates the 5-HT2B receptor and causes heart valve damage.
All of these designer drugs out there could certainly have off-target effects that would never be noticed unless a clinical trial was run (even then maybe not!) or unless enough users have enough problems that it's noticed (MPTP is great example).
Washington, on the other hand, appears to at least be funneling revenue soley into the health department.