Question : When the power goes down the 3.3v feeding the 2.5V regulator could decay at a little different rate than the 5v feeding the DSP I/O regulator. This might cause the enable/disable function of the 2.5 volt regulator to momentarily be enabled, which might put some voltage back on the I/O supply with the 2.5 V absent, or nearly so. It could possibly violate the restriction on 3.3V not being higher than core voltage plus 2 volts. Is this restriction an absolute? What happens if it gets violated on power down? It seems like the violation would be no more than a matter of milliseconds.

Answer : The C6201 does not have internal protection diodes, so if the power-up/down conditions have the opportunity to be violated, then external protection diodes should be placed between the voltage regulator outputs (as mentioned below).
Having the diodes in place should make it OK. Violating the condition by having the 3.3V on without the 2.5V does present the opportunity for the C6201 to be damaged, so including diodes is a must.