I think it might be less taxing (and have less impact) if we were to extend it.

Just a suggestion.

The priority also could be lowered too due to bugs that prevent apps from running and operating (which also impeded development of new features).

1. public interface ICallback extends Serializable

this seems ok for callbacks themselves.

2. public class CallbackStack
{

does not implement nor extend Serializable

But it is an ASO in hivemind with scope session...

<contribution configuration-id="tapestry.state.ApplicationObjects">
<state-object name="callbackStack" scope="session">
<create-instance class="org.trails.callback.CallbackStack"/>
</state-object>
</contribution>

which should suffice.

Did you try to add
public class CallbackStack implements Serializable ?