Could 8V of power be safely put onto the 3V3 input of an Arduino Uno?
According to the Adruino info page, and to the schematic linked there, the 3.3 volt terminal is an output from an internal regulator, and cannot be used as an input to power the board.  The schematic clearly shows the AVR powered from 5 volts.

According to that page, the board can be powered by 5 volts from USB, or by an external 6 - 20 volt DC supply.  There is a warning that the on-board regulator may overheat if the external supply is above 12 volts, and may be unstable if the external supply drops below 7 volts (so it should really be spec'd for 7 - 12 volts, not 6 - 20, in my opinion.).