Well, if there was some way of detecting motion ( increased alternator output, or some sort of switch on the accelerator cable linkage ), you could switch to running SVO when at idle. This would be the best way, because you could then stick a couple of 2 litre bottles in the engine bay.
This would be much more subtle than putting a tank under the back seat etc..
Of course, you could always remove the spare, and stick a doughnut tank in there !