I wonder if it's possible for transparent objects to cast full shadows Oo
If not, maybe you could leave it opaque, but add a shader to it with dithering. So it would not be half-transparent, but you will be able to hide pixels per pixels, a bit like the one that can be seen in the official TPS demo (but that one is
distance fade+``PixelDither`, not what you want but shows the effect). However I don't know if that's gonna affect shadows... you should give it a try.
I also thought you could check
Opaque Pre-pass. Unfortunately, it also modifies the shadow, i.e the transparent parts are also removed from the shadow :(