To me, the easiest way to replace fractions like 1/3 with the prebuilt fractions like β is to start with a dictionary that maps "1/2" to "Β½", "1/3" to "β ", "1/4" to "ΒΌ", etc. Then use the Search and Replace action. Here's a small sample:

In this example, I've included only some of the vulgar fractions available; you can fill out the rest if you think this will work for you.

(I should mention that I took the easy way out in specifying the fraction-finding regex. Since the Unicode fractions are restricted in how many digits can be in the numerator and denominator, the regex should account for those restrictions. Mine doesn't.)

I don't recommend building fractions from the superscript and subscript characters. It involves a lot more programming and tends to produce ugly fractions.

By the way, if you want to enter the Unicode fractions as you type, you can set up Keyboard Maestro macros that do that for you. For example, I type β;1/2β and get βΒ½β. This sort of text substitution can also be done using tools like TextExpander, Typinator, or Appleβs own text substitution system.

This is not an answer to @Potts_Jeff's question, but I thought it worth mentioning in case people find this thread when searching for this kind of substitution.