This question has been answered by Bruno17. See the first response.
{"field":"test_single_select", "caption":"TEST", "inputTVtype":"listbox","inputOptionValues":"Test1||Test2||Test3"}
Thanks Bruno, I appreciate your help.
I'm a bit surprised that it's not enough just to specify the options in the component TV, but the explanation/clarification makes perfect sense...
Cheers.
{"field":"test_single_select", "caption":"TEST", "inputTV":"test_single_select"}
And just to verify, if the list is coming from a snippet, I can use
... ,"inputOptionValues":"[[getSubpageList? ...]]"}
@EVAL return $modx->runSnippet('getSubpageList',array('propertyA'=>'A','propertyB'=>'B'));
@CHUNK optionsChunk
[[getSubpageList? ...]]