You might consider having a good smith line the .410 barrel instead. It would probably be a much more routine and cost effective option. There are very good liners available, likely the equivalent of many after market barrels. The same smith could easily fiddle with the extractor and set the barrel up the way you want it for your favorite sights. I'd consider a decent trigger job.
Having seen this done with a really nice old Belgian double hammer gun, I would go this route too except that I might have it done with barrels turned down (or ordered) to size rather than liners. I once had a hankering for a Dangerous Game Double Rabbit Rifle in .22lr. I still like that idea. So, I bought Graham's(?) book and studied it for a while, but never did move forward on it.
The guy that did the .22 set up some eccentrics so that he could regulate the double. The book discusses this too. It's not so simple as sliding in a liner and some glue. But it seems very doable (says he who does no machining).
A .22 double built on a .410 - still sounds like heaven to me. Something that would be shot a whole bunch and really enjoyed. I need to revisit this. 
