There are some options, and more than existed just a year or so ago.
You can take the guts out of your current tach (including the immobilizer), hide it underneath the tank and run an aftermarket tach off of it. That's the set-up I'm running on the M1000SS (formerly an '03 M1000). Talk to Duck-Stew, who built my bike, re the specifics.
Alternatively, you can have Monstermash reflash your ECU. I'm not sure if just disabling the immobilizer re the reflash allows you completely get rid of your current tach. Check with him:
http://ducatimonsterforum.org/index.php?topic=15502.0If they make a Nemesis ECU (not cheap!) for the S4, that's another option. I know they're available for later models, but am just not sure. Anothony at desmoworks.com should be able to point you in the right direction.
It used to be that you could run a FIM ECU, but I think they're out of business and I know of only a few (two in the US to be exact) who had success with that route. At this point, I'd consider it a dead end.
P.S. I'm not surprised you're not getting your answers at TOB. After the mass exodus most of the community (and the knowledge base) ended up here, though some people still frequent both boards.