I think this will solve it: You could remove the dependence on
the original grid for the single-valued variable lvi by evaluating
it in the command. Use the grave-accent syntax,
let/unit="Sv"/title="Total
Net Transport" c = `lvi`
I started to write that answer as a response to Rebecca's question, but then I didn't know when you *need* to explicitly reduce a multi-dimensional variable to a scalar as you write above. For example, you don't need to explicitly reduce it in the following:
yes? set data levitus_climatology
yes? let mt = temp[x=0:360@AVE,y=90S:90N@AVE,z=0:6000@AVE]