Will an Early Access game go on sale?

Yes. I remember assetto corsa while being in EA gone down to 50% on summer sale.

According to past experiences with sales, the chances are next to none.

For example: I'm looking forward to buy Salt (33%), but I'm not sure if it will go down on higher discount.
EDIT: They plan to release the full version on 2015's Fall... Does that change anything?

depends on the game, but most of the time you won't see anything greater than 33% (there are a few rare cases of EA games in bundles)

also, something like Rust has NEVER gone on sale

Space engineers is on sale for 50% right now and its in early access.

I bought Grim Dawn in summer flash sale, 50% for 8 or 12 hours it was I guess.