Yes interesting...I may end up just offering a DC input jack and let people power it how they see fit...like you suggest or otherwise...thanks
To answer two questions, if you let them "power it how they see fit" and don't include a v.reg,
be certain they'll blow the whole thing. Risk goes up by 10X if you have a power jack on the
board. They can connect anything.
Batteries direct to microcontroller may be ok in a very carefully controlled situation and where
you're not using A/D converter, but risky for a general case. I think it's better to use a v.reg
and have a good overhead margin in voltage so it keeps running as the batteries discharge.
With 6 NiMH AA-cells and an LDO v.reg, the cells are depleted when down to about 1V, which
is where the LDO v.reg also drops out. 4 alkaline cells are also a usable choice.
NiMH AA-cells have up to 2500 mA-hr, the measly 9V batteries maybe 1/10 that. I might use
a 9V battery with a microcontroller operating with a 32-Khz xtal.