Can regular bike forks be modified to allow front drivetrain setup? Can regular bike forks be modified to allow front drivetrain setup?

Some bikes have front drivetrains instead rear drivetrains. The problem is that bike stores don't seem to carry forks that have a derailleur hanger. Forks don't always outlast the bike and custom building another fork is likely very expensive unless it's mass produced.

It's possible that you want or need a different style of fork. One might prefer metal instead of carbon fiber for safety and a larger room for errors, one might need a different style for panniers and fenders for touring and commuting purposes, one might need a different shape for fork lights or cameras, one might prefer mounting holes for a disc brake caliper, you nane it. 

Here are some bikes that have a front drivetrain:









Can you widen a steel fork from 100mm to 130mm to accommodate 8 through 10 speed hubs using the cold set method? 30mm of widening sounds quite large. It's almost three sizes up. I've heard of 10mm. What are the risks? Once you widen it, how do you install the derailleur? Can you tap a custom shaped steel bar and use it as a derailleur hanger?

You can find standard spacings for hubs and seatstays here:
https://www.sheldonbrown.com/frame-spacing.html
To cold set a regular fork by 30+ mm would no doubt weaken it far too much to use. The better starting point would be something like a 135 mm fat bike fork, common sizes are also listed in the Sheldon brown table you linked. 

Notice however, that three of the four pictures you found feature extensive modifications to strengthen the fork or supporting frame to handle the pedalling forces. None of them will have off the shelf forks, they will all be custom made, along with the whole frame; they have totally different riding positions to a regular bicycle. 

If you are serious about getting a bike with front wheel drive, it’s worth doing properly which would mean sourcing something specially designed for the purpose.