I've seen some very creative card "walls" in use - though I haven't seen a flat one yet. It's worth a try. :)

I have a bunch of questions about how you'll make it work for you. Would you lay a magnetic whiteboard flat and secure the cards with magnets? Will you use a regular table top? What will you use to keep the cards in place, yet easily movable when intended?

As you mention, card walls are all about visibility. Do you have a centrally located, visible place for the table where people will come to move cards? How will you increase the visibility?

I join jbosse, in wondering how you will keep the virtual and tangible in sync. Do you have an Agile coach or sm who will keep them up, or will team members take on the task?

Other creative methods I've seen when no common wall was available (none are magnetic so you have to use the index cards with sticky edges) include:

"found" walls - a large folding table turned on it side and mounted on another table

portable walls 1 - a rolling whiteboard (sometimes these are magnetic) that can shift around to be visible to different people

portable walls 2 - a piece of coated shower wall material that's lightweight and easily leaned against an available cube or window without attachment.

hung walls - a clothes line strung from one point to another, with the heaviest flip chart paper you can find pinned to it then taped together side by side

window walls - use a well-insulated window (a relight can work well) that doesn't accumulate condensation from weather changes for a see-through wall