Matt,
Like what MAA said, I think there's definitely a market for some of the more hard to find props... especially if they were made for the AS community. As well as all the WWII re-enactors that would be interested in having the 50 cal., there'd be tons interested if they were made for airsoft. One of the most popular AS guns out there in terms of reliable gearboxes, and good upgrades is the TM AK-47. Most AS gunsmiths remove the internals of the AS M60s and replace them with AK internals. The same happened with the M249 when it first came out, until they developed even better after market gearboxes specifically for the M249. Considering how much space is available in the 50 cal. internally, you could easily fit an AK-47 in there, plus a battery to power it (like a high volt battery for an insane rounds per second count). Add to that a battery opperated box mag that auto feeds the BBs into the feed tube of the mag and you're good to go.
Most of the cost would be in the initial R&D of the 50 cal. AS conversion. After that, you could sell them as kits or build ups. If you take a look at some of the exotic AS kits out there, like the the H&K KL-8, the MG-42, etc... their prices are easily in the $1000s. Now imagine selling a
50 cal. airsoft, with AK-47 internals, that's made out of hi-impact plastic to keep the weight low and cost reasonable. You'd easily get buyers... if nothing else than because no one else has made one, but also because it's such a f'ing cool heavy weapon, that uses reliable internals and is aesthetically accurate. You'd probably be able to make a minimum of 35% profit on each one. A lot of it comes down to how strong you'd be able to make the replica... and after hearing yours and Jesse's comments on R&D into high impact resins this all sounds very promising.
The MG-42 would most definitely come under the same criteria.
As for a smart gun... that might be a little complicated to make as an AEG. The motor that drives the gearbox in most AEGs (except the P90, FA-Mas, and AUG) resides in the AEG's pistol grip. If I'm not mistaken, the MG-42's pistol grip used in a smart gun, has been removed. We can't really do that with the AK-47. One option would be to have the motor hang down below the smart gun receiver, which will detract from the overall accuracy of the gun, or another option could be to use a P-90 gear box which keeps the motor parallel to the gearbox instead of diagonal which is common to all other AEGs, including the FA-Mas and AUG. Using a P90 will probably keep the overall aesthetic true to the Aliens smart gun.
This is all hypothetical of course... don't want to break board rules.

Does anyone else have "hypothetical" thoughts on the matter?
-Alex
p.s. Sorry for the rant.
"The M41-A. When you absolutely, positively have to kill every mofo in the room. Accept no substitutes."