Lots of people suggest soaking cocoa nibs in vodka for a couple days to extract more of the flavor and sanitize before it gets thrown in the secondary - why not soak it in imitation maple flavor? It contains 12% alcohol. I know a lot of homebrewers are purists, but I think imitation maple would do a great job at producing a maple flavor without having it just ferment out.
If you are soaking anything in a spirit for the purpose of extracting flavor, then the higher the abv of the spirit the more effective of a job it should do extracting flavor. Therefore vodka (40% abv) is more effective than imitation maple flavor (12%). Besides, vodka is neutral so it doesn't alter the flavor of the ingredient you are soaking. Plus the amount of maple extract you'd need to cover the cocoa nibs would probably be way too much to put into your batch of beer. P.S. If extracting for flavor, 5-14 days is generally better than a couple of days.
If you're looking for extra flavor, there are plenty of spirits that can provide that...Bourbon and Rum come to mind.
There have not been many users of maple extract on this forum to say whether it works to give a nice maple flavor, so I suggest that if you want a maple flavor in your beer, use a combination of the extract with vodka. I think @koopa is right that it would take too much maple extract to use it exclusively on your nibs, and I think it would give way too much maple flavor and mask the cocoa flavor too. If you use the maple extract in any form, please report back with your success.
Imitation maple is sometimes made with fenugreek. I have tried to use fenugreek in brewing to get maple flavor and it sort of worked. It added something maply, and also something sort of earthy and distasteful to my palate. If using your imitation maple for the first time in brewing, I'd recommend trial batches or else make a maply-cocoa tincture and first add to a few glasses of beer in different amounts to evaluate how much, if any, you want in your beer.
