I tried many different configurations last year... finally found that if I put the harness under my tekvest so the beacon sits between the front and back half, the strap doesn't dig into your neck and it's still to the outside of the armor zone.
As far as the polycarb roost guard attenuating the signal... Lexan is rather transparent to RF signals. Cellphones, computers, pagers, bluetooth devices, remote controls, even the beacon itself encapsulates antennas inside the plastic housings. Snow and moisture buildup on your body is going to have more of an impact on the transmitted power than the plastic roost guard will.
Just a side note, but have you worn the roost guard with a pack and everything? I find it to be quite uncomfortable with anything else on top of mine. A jacket I can deal with, a pack I cannot.