Tuning Software.

[Vertigo]

Also, licence it free-as-in-beer imo, with a commercial licence required for workshop use. Perhaps let it be used with X number of unique ECU's before it refuses to work without a commercial licence.

[Si]

This would be an interesting use case for a raspberrypi unit....

Could easily run software to translate between a users command and different ecus, as well as run a webserver over wireless or bluetooth. Plenty of open graphing and monitoring tools around. Running wireless would require network configuration but could be avoided by running a dhcp server...

Sergei's mention of scripting could easily be included, and possibly with a gertboard additional IO and sensors be accommodated. If the unit was real time enough things like having a boost sensor, and a air temp sensor could be combined to feed one value to a ecu... Or a bunch of other clever things....

[Si]

Oh and licensing. Hardware would be a couple of hundred dollars, and include the software free.
Commercial use paid for - easy to implement. If you were say dyno tuning would need something more substancial than a raspberrypi, and you would want it set up however you wanted without having to reconfigure someone elses unit. So sell a hardware/software system. For simplicity sake keep it ARM, although x86 would be somewhat easier to get a high performance system.