Sure it would be doable, but, the IS would require quite a bit of custom stuff to make it all work right... The cluster, for instance, is multiplexed and can't be driven from any ECU except the factory ECU... One reason why 9 out of 10 standalone equipped IS's retain the factory ECU...

The wiring is always a nightmare and trying to get the Aristo stuff to work inside an IS, well, its taking a engine/trans out of a car from 10 years ago and putting it into a car that has most of the latest bells and whistles and is still current with todays technology for engine, transmission and body controls...