I wouldn't consider 27" a restriction, a quick look at my catalog here and I see three ready built 27" wheelsets, prices range from $99 for a cheap Chinese steel set, to just over $200. There are no 700C road wheelsets below $250.
What I see as the middle or upper priced 27" set would be fine on just about any 27" bike, either would be at least equal to what it came with, although the upper end set is likely spaced at 135mm. The two upper sets use Sun rims, the one at $220 lists CR18 rims, the other doesn't specify.
The original wheelset on an early Trek 412 was usually Rigida 1320 rims with early, adjustable cone Sunshine Gyro low flange hubs, but I agree, during those days, substitutions when it came to various components was common due to high demand. Most bikes advertised the rear derailleur model, but many didn't specify the front. I also suppose manufacturers selling a bike to the masses likely would shy away from ultra light fragile parts. (Several aluminum cage front derailleurs come to mind off hand).
The Rigida 1320 was a soft rim, it wasn't a bad rim but there were lots of better choices back then for a bike that was likely going to be ridden on your average road here. I can't speak for all parts of the country, but most of the roads where I road back around that time were a mix between dirt, loose gravel, and oil and stone pavement. Only the main highways were concrete or smooth asphalt. It wasn't until the mid 80's or so when they started ripping up the oiled base roads and putting down smooth asphalt.
If you ever road on oil and stone roads, he know how hard it is to ride on, and how many flats it causes. The chipped stone they put down was at first like riding on loose gravel, then once it packed down, it was rough with lots of tiny sharp rocks embedded in the road. Narrow, high pressure tires didn't stand a chance. The problem nowadays is that many of those road that were paved with asphalt in the 80's are now in really bad shape, with lots of pot holes, lumpy patches, and open cracks.
A bike running an inch wide tire doesn't stand a chance. .
Years ago we replaced several of those Rigida rims every week for cracks, lost spoke eyelets, flat spots and even sidewall blow out. They weren't rim that instilled confidence when you pumped up a 1" tire to 135 psi. They also weren't a good choice for a larger rider. Very few bike owners wanted us to replace them with the same rim.
The 412 wasn't a low end model, it was just Trek's lowest cost model. By 1980, Trek had dropped its entry level models and for a while the 512 was the lowest cost bike they had. A guy who wanted a 'cheap' bike didn't go looking at a Trek, they bought Columbia, Ross, Nishiki, or any of the steel wheel Japanese bikes that were around then. The 412 was a good bike, I never remember any issues with that frame and thousands were sold. It was an excellent riding bike in that the geometry was a bit more relaxed and occasional riders took well to it. A group that likely accounted for likely 99% of all bike sales back then. I can count on one hand how many guys came in who were 'serious' riders back then, most were just buying a bike to ride, color and style meant more than form and function. The shop I was at back then sold a slew of brands, Trek was just one many. We likely sold 200 cheap, steel wheel Peugeot A08 or U08 bikes to every one Trek. Even Ross and Columbia out sold Trek. It wasn't a reflection on Trek, but it did show how people thought and spent their cash. Most didn't care if it said Trek or anything else on the headbadge so long as it had drop bars and looked good. Trek bikes that were on the floor ready to sell often sat there for weeks or months before that one person came in the door who wanted at better bike, then the problem was that those same people tended to look right past Trek, a new comer back then, and go for the upper end Raleigh or Motobecane for a few more dollars. Trek hadn't made a name for itself yet so there wasn't a huge cult following back in those days. So like many of the lesser known brands, they were a bit slower moving when it came to sales. As they gained recognition and a few magazines took notice sales become more frequent. Most of us never thought they would survive back in the day. If brands like Raleigh, Motobecane, Schwinn, and Peugeot hadn't faltered so bad in later years, I'm not so sure Trek would have become the well known brand there did.