A typical patient has no idea about the precise nature of the manipulations that are to be used - location, intensity, type of manipulation and so on. For chiropractic to be valid they need to be able to prescribe a specific treatment to a specific condition for a patient. If the prescription is valid then if the patient undergoes a different manipulation it shouldn't have the same effect. If it does then the idea that it was that specific manipulation is potentially invalidated.
You could actually even double blind it by having one chiropractor diagnose and prescribe treatment, and then have that treatment randomised so it was either done as prescribed or substituted for a sham treatment. That treatment would then be administered by a second chiropractor who was unaware of the diagnosis and not allowed to speak to the patient about it or modify the treatment.
Even if that was too complex or otherwise flawed a properly constructed single blinded RCT trial is still valid evidence, even if it's not the best of the best.
See: http://www.ncbi.nlm.nih.gov/pubmed/10519559
You could actually even double blind it by having one chiropractor diagnose and prescribe treatment, and then have that treatment randomised so it was either done as prescribed or substituted for a sham treatment. That treatment would then be administered by a second chiropractor who was unaware of the diagnosis and not allowed to speak to the patient about it or modify the treatment.
Even if that was too complex or otherwise flawed a properly constructed single blinded RCT trial is still valid evidence, even if it's not the best of the best.