"I'm curious to learn if an evening spent disassembling, fettling, lubricating and reassembly might result in smoother-functioning tuners."
Doubtful. My experience with cheap tuners, both sealed and open back, is that the bearing surfaces and the "slop" between the gears is their weak point. With open backed tuners the gears can often be brought into better alignment by tapping on the worm gear supports, and if the holes through the peghead for the posts are properly sized and the tuners mounted correctly the posts will have adequate support to hold the alignment reasonably well.
With sealed tuners both the alignment of the gears and the support of the posts rely on the cast housing and the tolerances built into it and can't be easily adjusted (or at least I haven't figured out a way to do it
).I use cheap open backed tuners on some cheap and cheerful instruments I build - stick dulcimers, cigar box ukes, formica bodied and experimental guitars and such, but with the thought that if the situation warrants they can be replaced with something better (open backed tuners with their smaller post holes are usually more easily replaced than sealed tuners).
There are some inexpensive "semi-brand name" open backed tuners that hold up reasonably well and are a similar quality as those used on the older iconic guitar brands.
I'm with you on holding the line on material costs to keep the hobby affordable. Still, good quality tuners are often the most expensive single component of the instruments I build. I can sometimes find them discounted on the auction sites or when a supplier is having a closeout sale. If someone wants me to build an instrument for them I have them pick and supply the tuners - they can choose the style and quality they prefer.
If you really want to use sealed tuners with cast housings, you may want to find some that won't require fettling or modification. Others may know better, and offer suggestions that will be more helpful.
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