I'm converting a mid-80's Miyata into a commuter, but do not have clearance to switch to 700 wheels. So, on beaded rims listed as 26' x 1.5", could I use a 1 3/8" tire?

Most 26x1 3/8 tires are made for rims that have a 590mm bead seat diameter. 26x1.5 or any other decimal are made to the mountain bike standard 559mm bead seat diameter. You could use a 26x1.25 tire, if you are looking for a narrower tire.
Sheldon Brown's Bicycle Tire Crib Sheet

+1

26X1-3/8 tires will not fit on the same rim as 26 x X.XX (any decimal size) tires. 26 X 1.0, 26 x 1.25, 26 x 1.5, etc. will all fit (although, as a general rule, I wouldn't recommend using a tire narrower than the rim).

For commuting, 1.5" wide tires (~38mm wide) tires are very good. 1.25" (32mm) wide tires would be ok too - 32mm is a very standard size for all-around riding. I use 700 X 32mm tires on my road bike.

+3.......ran into this when I was looking into some tires for a 3 speed I was working on..... mostly 3 speeds and old dept store bikes use the 26"x1 3/8" tires..... not interchangeable with any other 26" tire

You will need to get 26x1.25 to be sure you can check the metric sizing which would be 559bcd with 32-25mm width most tires should have both marked on them.

Also, the markings on the rim are not usually the actual size, but the nominal tire size. You can vary from that as long as you don't go narrower than the rim or wider than the frame will accommodate.

Got it...thanks for the replies!