Actually, any key that you don't use as a hintkey can still be used to narrow down the hints. I use lowercase hint keys, and I can hold shift and type the link text (as the matching is case-insensitive, but the labels are not). It works very well.
For aesthetic reasons I like to render the hint labels in uppercase. It can be done by adding text-transform: uppercase; to the Hint selector with :highlight
Also - if you'd like lettered keys, try setting the following option in your .pentadactylrc:
set hintkeys='ASDFGHJLK;"
You'll lose the ability to type the hints' text to hit them.