IPIP is possible on the SRX but not in cluster (virtual chassis mode)
Being able to setup a single or even multiple IPIP tunnels is not suffcient to be a member of the AMPRnet IPIP mesh. See what Brian wrote:
This is a mesh network, not a star, so there is no default route; you need a separate tunnel route to each gateway you want to communicate with,
Marius has written a very nice script to get it working on a MikroTik router, but most other commercial routers lack the scripting capabilities to pull this off on their own. Someone wrote a script to use a Cisco router but it requires a separate machine to run it.
Rob