The only requirements are the obvious ones: any aimable lamps (headlamps, fog lamps) must have a system for attaining and maintaining correct aim as specified in the regs. Any replacement lamp must provide at least all the required functions the original lamp provided, and all functions must comply with the applicable provisions in the regs. In your example of sealed beam sized headlights, you'd probably be fabricating bezels and fillers and structures, but the actual headlight mounting bowl with its retainers, springs, and aim adjusters you'd probably buy (as you say "any manufacturer's adjustable bezel"). So long as you buy a good one (original equipment of whatever vehicle, reputable aftermarket), so you'd be offloading the engineering -- just make sure your bezels and structures don't prevent the mount/aim hardware from working the way it's designed to work.
(Likewise, there's nothing stopping you from putting Chevrolet lights on your Subaru...as long as they're mounted and aimed and performing according to the regs and they provide at least all the same functions as the original Subaru lights did.)
In practical terms: let's say you want to make a modular tail light for...I don't know, let's say the last-design Ford Crown Victoria. That car's original tail light on each side provides stop light, tail light, rear turn signal, rear side marker light, rear reflex reflector, and rear side reflex reflector functions. So your new tail light has to provide all those same functions. At the Federal level it's not OK to "export" functions, like you can't make a taillight that doesn't have integral side and rear reflex reflectors and ship them with separate reflectors. But at the level you sound like you're talking about, building lights for your own vehicle, not creating a business to make and sell them, there's nothing stopping you from exporting functions: originally integral side markers, now separate, or vice versa? Fine, as long as the vehicle has all the functions it's supposed to have.
Whether you're doing headlights or various other exterior lights: use good quality, compliant light modules/units. There is good variety of good product on the market; unless your desires are really exotic, you should be able to find what you want easily enough. But there's also a lot of garbage out there, so tread carefully.
Use light modules that are actually designed to do the respective jobs you're getting them for. A stop light has to be a stop light, engineered and built and certified/approved as such -- not just a light that lights up red. A rear fog light has to be a rear fog light, not a stop light hooked up as a rear fog light, even though they're both bright red lights, and so on.
And make sure you really know and understand the rules and requirements that apply to the lights and vehicles you're working on. Don't guess or assume anything; don't think that a quick reading of your state's vehicle code provides all the info you need.
Ask questions along the way.