We have a stainless arch with 3 solar panels on it and none of it is grounded. You may know also that the masts and rigging on an SM are not grounded either.
The only ground (Bonding) connections on our boat are metal items that are in continuous contact with salt water i.e. the main engine, generator, head pumps, salt water manifold. The reason for this is corrosion prevention, nothing to do with electrical protection. I don't think it's necessary to "ground" the solar controller; it's not in contact with salt water and therefore bonding is unnecessary.
Remember this "ground" is only for corrosion protection, it's not like the ground you have in hour home electrical supply...which really is ground (stake in the earth).
G21 Opua NZ