FYI Meridian tandems with 160mm rear spacing use 127.5mm BB axles, not the "standard" 118 that is commonly used on other tandems with 145mm spacing. (Meridian used the same rear spacing as Santana IIRC.) Using 118 will result in the poor chainline you mentioned.
I'll throw this out there: you could consider running square-taper cranks and matching high-quality BBs (e.g, Phil Wood or Hadley). I have these on my Santana MTB tandem and they are indestructible so far (had them since 2001 and used on several tandems).