I, and I suspect, many others have thought about it, but the mechanics of building it, and sealing the gap in both directions has always been insurmountable. Plus, you still need enough foil in the water to prevent leeway, so the added complexity does not get paid for by reduced wetted surface.
There appears to be two different steering scenarios for a shunting multihull.
When not moving, or very slowly moving, through the water the steering effect of a water foil is minimal or non-existent. Steering is effected by sail balance (i.e. schooner rig), very large rudders or a steering oar. This is not the total picture but approximates the problem.
Once there is reasonable speed through the water, steering can be very effectively carried out by water foils.
In the latter case an articulating lw hull might act as an inefficient but adequate water foil. A benefit of such a foil might be greatly reduced drag from other foils such as rudders and removal of the risk of rudder failure as speeds and forces increase.
I assume that someone has tried this. Can anyone comment on the results or point me to a source of data please.