dhenry:
Straight out, it is unlikely something like this can be used for a device needing a constant voltage.The topology, however, is likely a fly-back smps - assuming that it is isolated. That means the led string is floating above a current sampling resistor that controls the output voltage to maintain that constant current (thus constant voltage over the current sampling resistor - likely via a zener + optocoupler).
So all you need to do is to identify that resistor and put in place a divider. That resistor is very easy to identify: it must be between the cathode (led-) and the ground.
After that, you have turned this constant current regulator to a constant voltage regulator.
I have a similar problem and try to use these power sources. Little bit wiser after reading the thread I bought 12V LED drivers,the no power drain voltage slightly exceeds this limit with 13.5 volts. I did identify the resistor between ground and cathode, however would need little bit more info what divider to use (my learning curve is steep, but really not that I know a lot). Are you reffering to simple 2 resistors like explained here Voltage divider - Wikipedia ?