Thanks to you both, Toxaris and DiapDealer.
I included the code for ol.loweralpha in my stylesheet as directed, and when I replaced <ol type="a"> with <ol class="loweralpha">, the lettered lists displayed perfectly on the NOOK Color and on the iPad -- a great solution.
However, as DiapDealer pointed out, these kinds of multilevel lists aren't supported on the Kindle, so the choice is either to have lists with the same "type" of ordering, differentiated only by indents, or (the option I will go with) an unordered list below the ordered list, as the next level, on the Kindle. Because so many ordered lists in original manuscript should technically and logically be unordered lists (in my experience), this isn't as much of a stretch as you might think.
But, I am very grateful to know that I can have lettered lists when necessary, and that when the Kindle does support the "type" attribute for the <ol> tag, I'll already know what to do!