Just to add to my post further up this topic…
…I have discovered that many more sites are capable of working in this way. I usually check the names of the character form fields and, if they are named “char1”, “char2”, etc., I add them as custom fields. The problem has always been where the form fields aren’t named that way.
Yesterday, I found that you can use the label text rather than the form field ID or name, e.g. “Please enter the fourth digit from your passcode”. In this way, I’ve been able to set up a few online banking vault entries to log in completely automatically, rather than requiring me to enter particular characters.