http://www.counseling.org/resources/aca-code-of-ethics.pdf
C.3.b. Testimonials
Counselors who use testimonials do
not solicit them from current clients,
former clients, or any other persons who
may be vulnerable to undue influence.
Counselors discuss with clients the
implications of and obtain permission
for the use of any testimonial.

How do you interpret the last sentence of the clause?
How does one obtain permission for the use of any testimonial if one doesn’t solicit it?

I think, maybe, you can provide the opportunity for feedback without asking for it.
http://www.genbook.com/bookings/slot/reservation/30106446/158594930/158558469/1414735200000?bookingContactId=1122136237
this site demonstrates what the reviews it generates looks like. I booked with this Chiropractor and was asked a few questions via email after the meeting. It was a useful brief survey. One of the options, I think, was to leave feedback. If using something like this as a therapist, you could even link to the ACA ethics language from their survey (fine print).

Another option, which I think works really well is this….
http://www.virginiadetweiler.com/testimonials.html
The therapist makes note of the ethical dilemma while offering a terrific alternative.