Hi there -

Has anyone got any tips as to how to cue (or even describe) the sling systems that get used in exercises such as quadraped / pre-swimming?

I don't. This sounds like a question for Elizabeth Larkam or Wendy Leblanc Arbuckle.

I'm not sure if I'm answering your question or not, but I'll try. What about taking a theraband or a braided towel, placing it under the client's ribcage, and creating the sling yourself? You could try cueing the idea of lifted support at the base of the ribs with every exhale-- the client never rests on or falls into the sling, but she's also not trying to actively round away from the sling. Does that make sense?