Yes. But for this purpose, your class and all members should be static/shared. You don't need to create an instance of it then, you would just call the method that gets the boundary.
And, you can just store the ObjectId of the appended region/polyline in a static member from the ObjectAppended event handler, and you don't need to use EntLast() at all.
Lastly, you should use Try/Finally, invoke the command in the Try block, and remove the ObjectAppended event handler in the Finally block.