OK, I've some bad news. It appears the way plug-in is implemented is not compatible with the way list chooser in vRO client works, so you won't be able to use filtering capabilities of the list chooser.
The part of the plug-in code responsible for object searching assumes the query string provided by the user is an OData query. For example, the following scripting code shows how to enumerate all host machines whose HostName property is equal to AppServiceHost:
var all = Server.findAllForType("vCAC:HostMachine", "HostName eq 'AppServiceHost'");
for each (var item in all) {
System.log(item);
}
At the same type, list chooser in vRO client is not aware of OData; it just filters by a simple text being a substring of any of the object's property values. In the above example, in the chooser you would enter AppServiceHost and expect to get the same objects returned by the scripting above.
Unfortunately, this different behavior of plug-in backend and list chooser means the filtering for vCAC objects won't work in all scenarios. If you provide an OData expression, the plug-in backend will work, but the list chooser won't; if you provide a simple string, list chooser will work but the plug-in backend won't.
For filtering to work properly, the plug-in searching code should be implemented in a bit more intelligent way - it need to detect whether the passed query is an OData expression or not (eg. by checking for some prefix or trying to parse it), and then if the query is a simple string, the plug-in can construct a new OData expression utilizing this string; for example it can check if any of the object's properties contains this string.
