Great suggestion. This is one of my design pet peeves for anything related to banking / bill payments numbers that are long and unwieldy to type. The (eg. invoice) number 563466130 in base 35 is "9bh18y". The same number as 9BH18Y and so much easier to type. Base 35 for sha256 would however add 11 additional characters compared to the sha1 output length tho.