Ok, here is a weird suggestion but it might just work! Take a bottle of lager, pour it into a container, whisk until all the bubbles are gone. Now pop a lid on it and put it in the freezer overnight. The water freezes but the alcohol doesn't. Take the container out, attach it with a whisk or blade or something to make it into slush and separate the ice from the liquid. The liquid is now concentrated lager you can use in your fudge!
This is pretty much the process used in making ice beers, only the liquid is put in containers, shipped off to locations, reconstituted with fresh water and then force carbonated to give you those 'wonderful' ice beers you see in bars. But the point is you will have concentrated lager flavouring, much better than using a lager malt extract kit as that is unfermented sugars.
Now an obvious point to make here, you will loose some flavour in the process as your not using very high tech gear. Also the higher the alcohol level in the beer the more residue liquid you will have. So avoid 3.5% crap and go for a 5% + lager. Even better, try a German Oktoberfest lager at 9%!
If that doesn't work, try simmering the flat lager on a low temperature to reduce the liquid to a concentrate. I suggest you play with the ice beer method fist because you are replicating a process used in 'brewing' rather than some weird cooking thing.
Final thought, lager has a very light flavour. American lagers such as Budweiser use rice and other adjuncts to lighten the body and flavour of the beer while maintaining the alcohol content. This means they have less flavour in. Make sure you only use a lager which has ONLY malt, water, hops and yeast. That will give you the most flavour possible to work with. Having said that also try a bottle of Stout. Not that watery Irish stuff that sponsors everything under the sun in order to keep its market position and high sales, but a real roasted barley flavoured stout. That has tonnes of flavour right there that has a higher chance of coming right through in your fudge.
I hope this has helped.