Yep! Saw one at the london boatshow. It was on a mercury engine, altho it wasn't a Mercury conversion. I haven't got an outboard so didn't get any details. I know this ain't any help but its a start. Do a search on the web, that should turn something up!
This artical was on a 2 stroke, I would be careful, have had problems! will not go any further than that. 4 Stroke has been done, not going futher than that, Will post more info when things happen. I know that 15 hp are run on vapour, this isn't hard to do! The Italians who were saying they had the engines running have had major problems, majorand have pulled all there larger LPG outboards off the market.
Contact Calor Gas they may have more info