Ok I think you've all convinced me to give it a go with the wash/soak+dry. I'd obviously love to be able to machine wash it anyway so if it works that would be good. My only worry would be that the needed shrink in overall length and girth is incompatible with the available shrink in the arms (i.e. if I shrink to a good fit overall, the arms are too short, if I shrink only enough so that the arms are good the rest is too large).
I'm normally an XL in IH, but with their listed measurements pre/post soak, the XL would end up far too small in the chest.
Oh well, I guess that's the fun in experimenting, it's not like this thing was expensive or anything