I see that the tiny26 has a internal gain amplifier for the ADC. Does this amplifier reduce the input current requirements of the S/H?
The datasheet is rather vague on this. According to the datasheet, the input resistance of all AVR ADCs with or without internal amplifier is 100Mohm and they simply neglect to mention the input current requirements of the S/H.
There is also the question regarding the input leakage current. The datasheet does not indicate if the input leakage current (1uA) specified for normal I/O pins is also present for ADC pins.
Assuming the input leakage current is well below 1uA for ADC pins, then perhaps the tiny26 has a nice truly high input impedance ADC which would make it possible to get away without external OP amps.