Hmm...I am revisiting this now and, inexplicably, the streetWidth attributes are now being picked up. I'm not sure whether it is a system reboot, the update to CityEngine 2019 I performed or (let's be honest) some kind of user error, but at least the attributes are now coming through.
Ultimately, it doesn't look as though it's going to be of use to me, as the streets in my scene can be quite curvy, so any corresponding lot incorrectly gets picked up as a corner lot (as shown below)
Thank so much for your help though!