As an Ardor bundler, can you filter out which transactions you wish to bundle based on a specific criteria? In the Ardor blockchain, as a bundler you currently bundle all child chain transactions by the set fees your willing to accept on a child chain. But how can you bundle only transactions by a user that holds a specific asset or have a specific account property set under their account? 

I want to be able to bundle all transactions with a fee of 0 only if that specific user holds my asset or that I have set an account property on their account. How can this be done?
You can specify a bundler filter using the nxt.bundlingFilter property.  This is a Java class that is loaded by the bundler and will be called for each transaction to be processed.  The filter returns 'true' if the transaction should be bundled and 'false' otherwise.  Note that the filter executes as part of the server and thus can use any of the internal public methods to decide whether a transaction should be included.

Update: Ardor 2.0.13 now provides a custom bundler filter that allows you to select transactions based on an account property. See https://nxtforum.org/nrs-releases/ardor-v2-0-13/ for more information.