Decrypt encrypted RTP packet.

I'm stuck in a catch 22 trying to decrypt an encrypted RTP packet. How
or why it's encrypted is not important.

Ethereal is able to somehow identify the encrypted RTP packet as RTP
but it can't dissect the packet because it's encrypted.

I've created a plugin which will decrypt the packet but I can't perform
a 'Decode As' after the packet has already been identify as RTP by
Ethereal. The only way I can decrypt the packet is by disabling the RTP
dissector.

I tried to call the RTP dissector, after decrypting, from within my
plugin (call_dissector) but nothing happens. I suspect it's because it
has been disabled.

Is there any way I can decrypt the packet and then use the existing RTP
dissector to dissect it?