it is possible. the wiring diagram makes sense to me im an electrician tho, is this something you can buy?..
BUT!! im not sure it will really be worth the time it takes to do it.
depends how bad you want it really. but its would take a few good hours or more at an auto electrician and probably longer to diy. BUT!!!! if you can get IS250 wiring diagrams somehow. it will half the time to do it.
It helps, but it would be nice to compare it with a diagram of that for an IS model variant that has the auto folding mirrors as standard :D
Without it, my guess is you'll need an SPST Normally Closed automotive relay switch. The relay coil goes across the orange cable (pin 8). One of the relay contacts are connected to the power supply for the power windows, and the other to the green cable (pin 9). When the power to the accessories is turned off, but the power to the windows is still available - as is the case when you have just switched off the engine - the relay switch will close, causing the mirror's retracting motor to fold the mirror.
This is just my guess, but I hope it helps.
I've done something very similar to this on a Smart Roadster... I'll see if I can work this out before the Sydney cruise, as it's something I've wanted my car to do since I bought it, but never had time or a diagram to go off. Hopefully I can work some magic... fingers crossed.