You're quite right; what they really should do is produce AA-size Lithium-Ion batteries.
Not sure if it's possible though.
There have been some rechargeable Li-Ion CR-V3 batteries produced (the type the fL-36 and FL-50 can use), and these batteries only have 1/3 of the capacity of their non-rechargeable counterparts. IOW, not worth it.
As for Canon's noise reduction, as Canon's use CMOS sensors they need all the noise reduction they can get, far more than CCDs need.