It is theoretically possible, but whether or not you will find it useful is another story. You would need to run a GRE tunnel interface over the IPsec tunnel and set the GRE tunnel logical MTU high enough to handle a full Ethernet frame.
The impact of the fragmentation, reassembly, encryption, and Internet delays make the probability of application performance being adequate close to zero. If the application is so poorly designed it requires client and server to be on the same LAN, what is the possibility that the assumption of "on the same LAN" delays are not built into the protocols used by the application?
Bottom line: While it can be done, you first need to ask if it makes sense to do it! The effort would be far more productively invested into making the application WAN compatible.
Disclaimer: Are you sure you can't run the application by configuring the network to support appletalk routing? Does the application really run at the link level?
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
