Not all tasks are equal and most are dependant on previous and future task, so choosing 1 resource for it would in most cases be the logical choice. So why disconnect the WBS from the resource requirement, making the booking of such resource this complex.
The specify pattern option is (depending on the amount of task) to timeconsuming if you can only assign 1 resource that has other project tasks assigned already.

Would it be possible to add tasks to the schedule board or put in the possibility to change the WBS according to the availability depending on the chosen resource, say for instance in the specify pattern mode?

Please offer the ability to resolve scheduling/booking issues by rescheduling tasks.