Hi Bryan,
I know this is a bit of an old post, but in case anyone else had a similar question (like I did). Another way to accomplish this, especially if you want it to only allow numerical input is as follows:
In the body::esri:inputMask column, you would enter
0000000000+99
This will allow the user to enter up to 10 numerical characters on the left of the + , followed by two required numerical characters. Note, that the user can just type 123 (without putting in the plus sign), and it will be stored & shown as 1+23. The only thing to remember is that in the case of your first example for 0+01, the user would need to enter 01 instead of just 1, because two numerical characters are required on the right.