In the best of all possible worlds, everyone always uses the latest version of the firmware, but in a recent emcomm exercise, we realized that in the "real world" you bring what you have, and about half of the nodes that were brought to the exercise were v2 and half were v3.  In most cases, the owners just didn't get around to reflashing their nodes, but there might be some nodes that are high up on a tower or otherwise in a remote location where it is difficult to upgrade them to the latest firmware in a timely manner. 

I understand that a v2 mesh cannot connect to a v3 mesh over the air.  I tried to link the meshes via ethernet.  Here's what I have already tried (unsuccessfully):

1.  I tried connecting the WAN port of a v2 Linksys router to the LAN port of a v3 Linksys router.  The v2 router was configured as a "mesh gateway".  The v3 router was configured as a normal mesh node.  When I opened the mesh status page on the v2 mesh node, I only saw the v2 mesh nodes.  When I opened the mesh status page on the v3 mesh node, the ip addresses of the v2 nodes wifi interfaces appeared on the "current neighbors", but I could not connect to any of them.  None of the ethernet interfaces of the v2 nodes appeared on the "current neighbors" list.

2.  I configured a Netgear GS105E switch in accordance with the instructions published by AE5CA, at  http://ae5ca.com/?p=49.   I connected a Ubiquiti Nanostation NSM2 v2 node to VLAN 1, 2 and 10.  I connected a Ubiquiti Bullet M2 v3 node to VLAN 1, 2, and 20.  I tried various combinations of mesh gateway and mesh node settings. The results were similar to the test with the Linksys routers.  When I opened the mesh status page on the v2 mesh node, I only saw the v2 mesh nodes.  When I opened the mesh status page on the v3 mesh node, the ip addresses of the v2 nodes wifi interfaces appeared on the "current neighbors", but I could not connect to any of them.  None of the ethernet interfaces of the v2 nodes appeared on the "current neighbors" list.  When I connected my laptop computer to either VLAN 10 or VLAN 20, I could get to the internet.  I could resolve and ping servers that were on the internet from either VLAN but I could not ping the node that was on the other VLAN. The OLSR links/topology status page for the v2 node does not show any v3 node addresses.  The OLSR links/topology status page for the v3 node shows the IP address of the v2 wifi interface, but does not show the IP address of the ethernet interface.

Is this a "lost cause" or is there something else I should try?

Not technically possible, no matter what you try (connecting v2 and v3 nodes by any method). v2 used the OLSRD 'secure' handshake to talk only with a compatible node (v2 ignores packets unless they have the correct signature).

v3 does NOT use the OLSRD-secure functionality. OLSRD-Secure is a desired feature based on a secret key to authorize a node to connect--a default key is packaged in the v2 images (version 1.1.x releases of the bbhn images). We found, submitted, and fixed a couple of showstopper bugs before deciding it was in our best interest to pull olsrd-secure out (one of the factors to create v3).

We learned that olsrd-secure was not sufficiently stable and had been abandoned by the olsr.org community. WE will have to take ownership and make it usable-stable should we decide to do so.

It is in everyone's best interest to upgrade to v3 (version 3.0.x of the bbhn images) without delay.