Damon, I'm afraid there is something wrong with your specific implementation. I, personally, can't think of an instance where a router doesn't respond to an icmp request on any of it's interfaces. Others, please chime in if that's inaccurate.
At any rate, I mocked it up in GNS3 to make sure there weren't any funky software bugs.

Basic configurations were set up on the interfaces with each router having it's router number as the lo0
interface address (i.e. R1 - 1.1.1.1
). For verification, here's the rip routing table.
R1#show ip route rip
2.0.0.0/32 is subnetted, 1 subnets
R 2.2.2.2 [120/1] via 10.1.1.2, 00:00:07, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
R 3.3.3.3 [120/1] via 10.3.3.3, 00:00:26, FastEthernet0/1
10.0.0.0/24 is subnetted, 3 subnets
R 10.2.2.0 [120/1] via 10.3.3.3, 00:00:26, FastEthernet0/1
[120/1] via 10.1.1.2, 00:00:07, FastEthernet0/0
R1#
When pinging the multicast address of 224.0.0.9
, 3 replys came back; R2
sent a reply, R3
sent a reply, and R1
replied to itself because it is also listening for that address.
R1#debug ip icmp
ICMP packet debugging is on
R1#ping 224.0.0.9 source lo0 repeat 1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 224.0.0.9, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
*Mar 1 00:15:41.163: ICMP: echo reply sent, src 1.1.1.1, dst 1.1.1.1
*Mar 1 00:15:41.163: ICMP: echo reply rcvd, src 1.1.1.1, dst 1.1.1.1 <--- R1 Response
*Mar 1 00:15:41.179: ICMP: echo reply rcvd, src 10.1.1.2, dst 1.1.1.1 <--- R2 Response
*Mar 1 00:15:41.179: ICMP: echo reply rcvd, src 10.3.3.3, dst 1.1.1.1 <--- R3 Response
Reply to request 0 from 1.1.1.1, 8 ms
Reply to request 0 from 10.3.3.3, 24 ms
Reply to request 0 from 10.1.1.2, 20 ms
R1#
Pinging a multicast address will generate a response from participants listening to that address.
ip multicast-routing
enabled on your routers?