What a nice thing to do for your dear mother! I can't imagine a better place to take her than to WDW and the Wilderness Lodge!
What I would do if I were you is, about 3 or 4 days before you are scheduled to arrive, type up a nice fax with your date of arrival, your reservation number, etc.
Simply but nicely state it is your Mom's 70th birthday and you will be celebrating it while you are there. Let them know that in light of that fact, you would appreciate a complimentary upgrade upon check-in if one is available at that time.
Don't forget to make sure you state complimentary!
We celebrated my son's 13th birthday at the WL this past January. We had reserved a standard view room. I sent them a fax telling them about my son's birthday, and we were given a complimentary upgrade to a woods view, when we would've normally had a view of the service area.
The WL is so magical! The theming knocked our socks off!
Have a great time, and hope you get a little something extra for your Mom's birthday!