When using a 3-to-8 Line Decoder / Multiplexer such as the 74HC238, can I set multiple outputs (1-8) as high at the same time?
As opposed to only one output high at any given time, and the rest are low.
No. The purpose of a 3-to-8 decoder is to select only a single option. If you require more complex decoding then use a PLD or memory chip instead.