For this case, we are starting with a layout mostly done in L that has pcells but has no nets at all.
In XL, when using Connectivity -> Nets -> Assign to assign an instance pin to a net, the desired target (i.e. the second thing that it prompts you to click) is a shape or another instance terminal that has no net. In other words, "we want to connect this pin on this pcell resistor to this other simple connectivity-free piece of metal over here." Apparently, in the Assign command, the target has to be (a shape or via on) an existing net.
While VLS does make it easy to assign a net to the target before running Assign (set the net property on the target shape/via, or add a label and Extract Layout, etc), in this case, we don't really care what VLS thinks the net name is - we just want to see the flight and drag lines.
Is there a way to have Assign automatically create an arbitrary net with a unique net name on the clicked target, which would get rid of the need to take steps to make sure the target is a net before running Assign?