I'm at a site that has a dedicated line to a remote LAN for a sister office.
We would like to test access to some local resources to make sure that
office can see it, and we'd like to automate it. Ideally, I would like to
pass all packets from MYHOST destined for TESTIP through REMOTEGW.

My first instinct was to simply add a route like:

route add -net $TESTIP netmask 255.255.255.255 gw $REMOTEGW

.... but you can't do that because $REMOTEGW is about 4 hops away.

So, I though that I could use iptables to do it:

iptables -t mangle -A PREROUTING -d $TESTIP -j ROUTE --gw $REMOTEGW

.... but that didn't work either. For good measure, I made sure to add the
forwarding rule:

iptables -A FORWARD -d $TESTIP -j ACCEPT

.... but I still can't run a traceroute and see packets going via $REMOTEGW
to $TESTIP. Now that I am thinking of it, I'm not 100% certain this is
feasible (after all, there are several switches between $MYHOST and
$REMOTEGW).

Any ideas?