3d scan a hole, then print out a part that fits the gap. They used the free software 123Dcatch rather than a pricier laser scanner, so that may help you get a cheaper working model.

For doing a whole wall, it sounds like you'd need something more like a modified frostruder design. Check out the existing frostruder designs on raprap's pages. That's a syringe based method of extruding other materials. Should be possible to flip it to print sideways, and then use an air compressor and valve, perhaps to constantly deliver your medium rather that a limited volume syringe.

Are you looking at using something like plaster as the printing medium?

I reckon one of the big issues would be producing enough medium without it drying before it could be applied, as I think it would be quite a slow process.