Brian, you would have to write your own service for this. For example, if you have bunch of Things based on one Template, you can use GetImplementingThingsWithData to retrieve all Things based on that Template. But in your case, since they are different, you would have to treat them as separate/ individual Things.
If you go to Resources-> EntityServices from the Composer, there is a service called GetEntityList. That would give you a list of all Entities based on what you give as input (Thing, ThingTemplate, DataShape etc). You would then have to sift through that table to get your required list.
I didn't saw any best practices for this yet on ThingWorx, but I what I usually do it's to create a helper thing for this cases called:
And add any services for "WhateverMatter" on this Thing.
This is usually what you would do in other programing languages as Static functions, but as ThingWorx doesn't have Static services, you have to rely on Things with services, no ThingTemplates has this services are not Template of any child class.