Hi Nick,
Great post, and objective I've never tried to get out of using an Xtal before,
so hadn't previously considered some of the difficulties coming up with your own timing.

I know what you mean, playing with a phone that almost does everything doesn't motivate
me to build my own computers,
I do have to come back very soon for a project... kind of have to because I want the thing I'm making

If it is just a perfectly linear program that can boss the chip out of running any other program,
such as for a power supply or something... I think as things warm up you lose timing of the internal clock,
same as if there was voltage fluctuation.
but for the exercise... with the new rules as I understand them,
I would try a program that can interrupt itself with an output pin
connected to the portB interrupt pin. Then you could introduce a linear delay at the interrupt
vector, and control the delay before turning the interrupt back on.
I don't know if the jump to the interrupt vector is essentially a GOTO or not.
It jumps to vector 4, which would not be as fast as resetting the program counter to zero vector.
and potentially some inaccuracy caused if there's any variation in the response time to the interrupt.
Cheers, B.