Your use case is, in fact, supported: you need to use @Action(associateWith=...), see [1].
For example, suppose we have a "removeItems(…)" action:
public class Order {
@Collection
SortedSet<OrderItem> getItems() { ... }
...
@Action(associateWith="items", associateWithSequence="2")
public Order removeItems(SortedSet<OrderItem> items) { ... }
}
The Wicket viewer will then render the "items" collection with checkboxes, and any selected items will be used as the pre-selected set of items if the action is invoked
HTH
Dan
[1] http://isis.apache.org/guides/rgant/rgant.html#_rgant-Action_associateWith