Do you get to use workshop mana to pay the cost to flip on artifact creatures? My instinct is no...
Your instinct is correct. Turning a manifest creature face up is a special action according to 701.31b, basically just like Morph, and Mishra's Workshop can only be used to cast artifact spells.
@babau Torpor Orb is not better. With torpor orb, you have to cast dreadnaught at sorcery speed and pass the turn, opening you up to a Dack steal. With this, you can flash it in on their EoT and then flip it and swing on yours. This card is WAY better than torpor orb for sneaking in artifact fatties.
@babau Yes, but 1 non-shop mana on turn 2 should not be hard at all. The ability to flash a dreadnaught in on the opponent's EoT turn 1 is such a huge thing that it can't be underestimated. Dack stealing your DN is gg most times. Metmorph copying and such (fringe case) is also avoided with this. You still have to connect twice, and are open to a ton of artifact hate, so likely the format has gotten too fast and answer-heavy (force of vigor) for any non-lock artifact plan nowadays.