Mutual Encapsulation

In RFC 1326 Mutual Encapsulation Considered Dangerous.

I guess the author assumes that the tunnel ingress router DYNAMICALLY
finds the tunnel egress router, based on the destination address of the
incoming packet.

If the inside of the tunnel does use the heterogeneous routing protocol
from the outside of the tunnel, how can the ingress node find the
destination
of the tunnel packet ?

Is that possible ?

Jiwoong