I guess, you'll have to fake it. In your onPaste method:
1. Create 2 strings. One (s1) from the beginning of the content up to selectionStart. One (s3) containing the original context from selectionEnd to the end of data. This effectively snips out any selected text, which is what a paste would do. If there is no selection, selectionStart and selectionEnd should be the same, which is the current cursor (and presumed paste) postion.
2. Now you need to obtain the paste value
string (s2), which should be in the clipboard.
3. Put them all together: s1 + s2 + s3 and
you should end up with your post-paste control value.