I have a DSA-815TG so also have an interest in this.
It is my impression that early model DSA-815 used license codes that were restricted to upper case and numeric characters. This is the restriction on codes for the DS11054Z scope.
Like others, I found that license code generated on line were not valid in the current firmware. I was interested in advanced measurements after the option had expired and I requested and received a new trial from Rigol. The official trial license code includes upper case, lower case, numerals, and a period. It seems that the allowed character set has been expanded.
The license code is still 28 characters.
Andy