Can this 800 MHz iMac G4 boot into Mac OS 9?
This is going off of memory so unfortunately, I don't have a reference.

But the partial serial number that you provided QT219 gives us a clue as to when it was manufactured:


QT - I believe this is the device (i.e. iMac), but it's not really relevant
2  - The year it was manufactured.  0 = 2000, 1 = 2001, 2 = 2002, etc.
19 - Week in which it was manufactured.  19 = 19th week.


So, just looking up the week numbers for 2002, we can deduce that the iMac was manufactured sometime between May 6th and May 12th, 2002.  That model, M8535LL/A, was made between Jan. 2002 and Feb. 2003, is the model that can run Mac OS 9