InDesign help with GREP: superscripting a letter within a word

Sure, that can be done. Make sure you have a superscript character style already set up as desired, and in your GREP styles, use the following:

(?<=M)c

note that this will catch ALL instances of the following letter pair: uppercase M, lowercase c (Mc) but that combination - in English - is only really used in names, and not mid sentences, such as comcast where both letters are lowercase. However, if you want to ensure this only affects the start of words that have Mc, use the following GREP code:

(?<=\<M)c

I currently use similar GREP styles like this to sanitise databases where - while the names had been title-cased, they'd done so without taking into account names such as:

McMurray;

O'Driscoll;

I've written about it in more depth here: https://colecandoo.com/2012/08/08/no-fills-grep-styles-part-1/