I'd say the problem you're going to face, is that Dan's leather choices aren't especially "weak" when hit by water...On some leathers, thinner, more porous types, you can reduce maybe a half size to a size by soaking and drying...But the cowhide that Dan uses isn't especially open to water, which means it won't shrink all that much...I'd say you could still try it....You'd have nothing to lose..But....Be prepared for not much change at all...Maybe a half size at best?
What you could try is building yourself up to fit it? More chest, bigger shoulders would make the fit smoother and snugger, and perhaps pull the arms up a small touch too? But that would take about 2-3 months of solid diet and training...Not impossible, but would work well enough if you had the time?
That, plus the jacket shower? Might work enough to fit it well?