There is no better way to dynamically determine the tests ready for run, than using a custom property of the test. However it is not possible to edit the properties of multiple tests at once. You will have to this manually, editing one by one.
The other way can be using a script to edit the JSON of each of .tstest files (not recommended) and add the following line:
"CustomProperty1" : "Ready",
like in your case, but this way you risk to corrupt your existing tests and experience additional problems.
Thank you for your understanding.