I prefer the simplicity of a couple of diodes, and place them right in the output leg of the opto switch. 5 minutes and 2 50 cent Radio Shack diodes (1N4007) will get the job done. Properly chosen, there's no capacity issue, and I've never heard of one failing for any reason.

Of note, either way you go, diodes or bridge, when you're done and testing, the circuit must be live, under load. Otherwise you'll get wacky voltage readings for sure!