The reason I swapped away from "When you play this, if you played this from your hand, trash it." for Halfpenny is that you can actually immediately play it from the trash pile after the trigger, making it a cheap Silver. The cleanup delay is necessary to prevent reuse, not too unlike the cleanup delay on scheme is necessary to prevent action reuse.
Well, in Halfpenny/Masterpiece games, you could shorten it quite a bit. "Setup: Choose a player to go first. That player wins the game."
This rubbed me the wrong way, and yet I could not think of a pithy argument that would demonstrate why you were wrong.
So I spent the last two hours on the floor of my room playing a Solitaire game against myself. I forced Halfpenny, Masterpiece, and Highway to be available, and allowed the other seven to be randomly selected from Base, Dark Ages, Hinterlands, and Seaside. The shelter roll came up Estates, Al opened 4/3, and Bo opened 5/2, which I'm not entirely sure was the best outcome for Bo.
The board was Highway, Festival, Pillage, Margrave, Mystic, Wandering Minstrel, Marauder, Gardens, Masterpiece, Halfpenny. Al spent all the Halfpennies he could on Masterpiece turn 1, as you suggested, but could not piledrive the Provinces in a sufficiently timely manner with Margrave Big Money. Bo persevered in spite of rough luck, his 5/2 Margrave missed the reshuffle, among other things, although Bo and Al started getting similar luck after the early phase.
The reason it took 2 hours is because I logged all the moves by hand. I can transcribe them later if you want to see, although I am too tired to do so now.
I feel with a minimal amount of restriction on the kingdom I was able to find a board that had a healthy Masterpiece-Halfpenny interaction, so even if lots of them don't work out that well, it's at least not a no-brainer all the time for player one, and I don't think it is more of a detractor 5/2 Witch, which doesn't even require a card combo.