No Keep-Alive feature is planned for the reason noted in the following thread, but the 3.1 Reconnect feature should be of some help. Re: Reconnect/Auto Reconnect
The link you reference was announced in this forum by MinChen in late 2010: SQL Developer Keep-Alive Extension Created!
As far as I know, it is probably a good option if you find 3.1 Reconnect to be inadequate. Reconnect will be improved, however.
SQL Developer Team