Can I use an Olympus FL-50R wirelessly, via a Canon 5D Mk II, with a third party trigger ? If so, which make of trigger would be best. Manual only is fine.
Yes, nearly any manual-only radio trigger should work fine between the Canon 5DMkII and the FL-50R, as both the four-thirds flash foot and the Canon EOS hotshoe are iso-compatible, so the center sync pin signal should work just fine.
If you have any kind of flash you can put on top of the 5DMkII, you could also use the built-in "dumb" optical slave mode of the FL-50R to trip it remotely, but given that the 5DMkII doesn't have a pop-up flash, this may not be useful advice.