I think that's right, looping an output into an input works like this: Let \\(f : X \otimes Y \nrightarrow Z \otimes Y\\). Tensoring and composing with cap, cup, and identity profunctors allows you to feed the \\(Y\\) output into the \\(Y\\) input, and get a new profunctor via the composition rule. So if \\(f^\star : X \nrightarrow Y\\) is that profunctor, then it's definition is

\[f^\star(x,z) = \begin{cases} f((x,y),(z,y')) & \mathrm{if} \ y \le y' \ \text{for any} \ y,y' \in Y \\\ \mathrm{false} & \mathrm{otherwise} \end{cases}\]

So basically, unless \\(f\\) requires less \\(Y\\) than it produces, you don't get a non-trivial profunctor \\(f^\star:X \nrightarrow Z\\). For any pair \\((x,z)\\), \\(f^\star\\) would return false. If \\(f\\) does require less \\(Y\\) then it produces, then it reduces to asking whether you can get \\(x\\) given \\(z\\), so you can simply forget about \\(Y\\) inputs and outputs.