You're halfway to answering your own question. In fact I think you actually know the answer
Assume the galvanic cell made by your hull and the quayside produces 0.3 volts. With no AC messing up the GI this will register on the GI meter as 0.3 volts. The GI meter shows the voltage across the GI. It does *not* show the current. But the diodes in the GI will not be conducting so no current flows, therefore no galvanic erosion. With a direct connection (i.e. no GI) this will produce a current that depends upon the resistance of the complete circuit, i.e. from hull through earth bond, back the neutral/earth bond (possibly at substation), back through the ground to the quayside. It is the current that produces the erosion.
So with a GI there would be no erosion, as there is no current flow, as the galvanic cell made by the hull, quayside and water doesn't produce enough voltage to "breakdown" the GI.
The meter reads the nett DC voltage of 0.3 volts.
Now if there is sufficient AC voltage (be it 50Hz or RFI or whatever) so that the GI is forced into almost permanent conduction, the nett DC voltage across the GI remains at 0.3 volts. But as you quite rightly state this is now superimposed on the AC signal. And as the GI is now conducting (due to the AC), the superimposed DC voltage causes a current to flow, which is the galvanic corrosion current. The meter continues to show 0.3 volts which is correct. But there is now a current flowing at DC.
So, although you didn't actually say it in your post, yes, you are correct
Gibbo
That's perfectly feasable and possible. If it does indeed do so then you can rely on them with confidence. But I seriously would have my doubts without making tests and measurements or seeing the schematic.
Gibbo