I can not ensure it (I don't remember exactly, I should verify), but I think that in the old Intel and Philips 8051 family data sheets is specified that you can drive a XTAL1 using another XTAL2. It doesn't seem to be using a chip outside the guaranteed parameters.
There are, indeed, such references. However experience show that the faster the chip gets (not what frequency you use, but the chips max speed) the more problematic this becomes.

No one guarantees that software setting an int. flag causes the interrupt to be called, in the datasheets.
Oh YES, read "the bible" it states that the flag, not what caused the flag to be set generates an interrupt.

Erik