[michaelt]

Presumably base 32 means 26 letters + 10 digits - 4 banned letters

So adding an excluded letter is not easy.

[sokoloff]

Why not use base-31 and (optionally) more characters? (Or go upper and lower or add a symbol if you had to stay with a fixed-size and base-32 for some reason)

[chipsa]

Because Base32 is just bit shifting and then converting the 5 bits into a char, and vice versa. Doing Base31 requires base conversion.