Is the username 35 character length limit configurable?...

Try using the Add User smart service to create a user, which should allow you to go up to 255 characters

forum.appian.com/.../Add_User_Smart_Service

Thanks, Tim! Any reason why adding a user manually has a limit, if there is no actual limitation on the back-end?

Thanks a great question, I'm not entirely sure on the answer to that, but I did report this to our product team with the reference AN-41747

Jason, no special reason other than keeping the field small for presentation/display purposes. This is a hard-coded value (maxlength="35") in a JSP (adduser.jsp) within the product.

I would ordinarily expect that validation on the designer interface would behave the same as validation on process smart services. It would be great to have more consistency (this isn't the only instance that you need to create a process in order to do something like this due to unnecessary validation.)