Neophyte19 hours ago, DamienC said:my initial question is about 'filtering' the result (fields=)
Whoa, I totally went into destructive teaching mode there and didn't even answer your question, sorry about that. Still applies and sorry to burst your bubble. Yes, REST APIs are very versatile, but they are still built by humans with certain intended uses in mind. Unfortunately, when the filtering capability was built, the human either didn't envision (<-- ha!) the need to filter by a specific property/value combo or they did think about it but didn't deem the need worth the work it would take to implement. All APIs have drawbacks and inconveniences, LM's is no exception.
Now on to your original question:
I haven't found a way to specify that only a certain property is returned. The best I've been able to do is pull the entire systemProperties and filter after pulling it back. The way i look at it, filter and field are a convenience if they work for a given situation, but if they don't you just have to do it the inconvenient way: slice and dice the data after pulling it back.