Optocoupler driving 300mA continously (or use darlington topology?)
The opto-coupler outputs don't need to switch the 300 mA current directly.  Use a transistor or two to amplify the few mA thru the opto to the 300 mA thru the LED string.

In this case, I'd probably use the opto to switch the gate of a FET.  Something like this:



You didn't say what the LED supply voltage is, so it's up to you to make sure that the opto output can withstand it when off.  You also have to adjust R2 allow enough current thru it to raise the FET gate to 12 V when the opto is on, but not waste too much extra current.  I'd aim for the zener drawing around &frac12; mA when the FET is on.