Off the top of my head? Make a little gadget that’s an inductor and capacitor, in parallel, tuned to 60 Hz (i.e. a band-stop filter) and, in series with that, a Y capacitor. Wire up this gadget in place of the Y capacitor, so you end up with two of them (line to output negative and other line to output negative, perhaps). Or maybe you just have one, and you connect it between the normal pair of Y caps and the output. It will have very high impedance at 60Hz, enough impedance from DC to a few kHz to avoid conducting problematic amounts of current at DC or various harmonics, and low enough impedance at high frequencies to help with EMI. It might need a couple types of capacitor in parallel in the band-stop part to avoid having the high-frequency impedance of the presumably large-ish capacitor in parallel with the inductor being a problem, and it might be an interesting project to tune it well enough to really remove the annoying touch current, especially if you believe in 50Hz and 60Hz operation. Maybe a higher order design would work better, but the size would start to get silly.