Is it feasible?  Sure.  Is it correct?  No, and there is a specific statute telling you what is.  Is the procedure convenient?  No, but if you had started it in January you would have the information well before October 15.

As usual there are more details missing here, than provided.  The mother would have to receive notice about the guardianship appointment.  So someone knows where she is, and whether she is living on Skid Row (zero taxable income) or filing jointly with a millionaire husband (taxable income in six figures).  If someone wants to take on the responsibility of guardian, that comes with a fiduciary obligation to make sure the kid's finances are administered correctly.  Should the guardian pay a lot more tax than might be necessary, by using his income when that's not what the law requires?  Or pay a lot less tax, when that shortcut isn't permitted either?  Remember, we just fill out the forms, we don't make up the laws as we go along.