I agree the makeup would've probably been simple, like the Mudd's Planet androids -- which arguably were a prototype Borg Collective of sorts, a group of individual bodies controlled by a single central mind. The menace would've been portrayed more conceptually and less visually. At the time, the idea of an assimilating collective would've been seen more overtly as a metaphor for a Stalinist police state and creeping authoritarianism -- which was still something of a fear in 1988 when the Borg were created, but the fear of being absorbed and replaced by technology, while present in the '60s (see "The Return of the Archons" or "The Ultimate Computer"), was probably less immediate than it was in the late '80s. So the technological side of the Borg threat probably wouldn't have been played up to the same degree.
Although I'm forgetting that the Borg weren't originally portrayed as an assimilating collective; in "Q Who" they were only interested in our technology and not in organic life forms at all (sort of like a cruder version of V'Ger, which "assimilated" ships and space stations and planets it was curious about but dismissed the "carbon units" within them as irrelevant). The idea of humans and humanoids being assimilated and turned into Borg wasn't featured until BOBW and didn't become their defining trait until First Contact
(since in most of TNG, Borg drones were portrayed as blank slates with no prior identity, the products of Borg incubation chambers). So the psychological horror aspect of the Borg, the authoritarian symbolism, was a later addition to the original concept, which was more firmly grounded in the fear of technology taking over.
But I guess that's kind of what I'm saying -- that if the Borg had been created by the '60s writers (which isn't implausible, since the concept is sort of a hybrid of Landru, Mudd's Planet, and maybe Nomad), it would've been more likely intended from the start as a metaphor for Communism (or rather, the brand of authoritarianism that misleadingly labeled itself as Communism) more strongly than as a metaphor for technology taking over. Or at least it would've been more a mix of both from the start.
In any case, yeah, I think the costume/makeup design would've been something simple and easily mass-produced. No elaborate prosthetics. Just pale skin, metallic jumpsuits, maybe fairly basic helmets, maybe some kind of small, standardized bit of tech that symbolized the interlink like the blinky amulets on Mudd's androids.