I've cleaned it out initially and will give it a good swilling today with peroxide. I suspect the loaner hadn't noticed it and it's been full of gunk for weeks. I mentioned it to her and she was completely unaware of it.
I suspect the lameness may be caused by infection: there appears to be some of the black pus stuff you find in hoof abcesses in there. Unless it's the thrush. Presumably, if there is infection, it is effectively laminitis since the lamini will be inflamed. The damage is on both front feet, areas about 1" long and a similar depth at about 10 and 2 o'clock, if the toe is 12.
We had shoes on her about 2 years ago (she's normally barefoot but was going to do a lot of roadwork) and she instantly went dead lame, like laminitis lame in her front feet. The farrier is a very good one and hadn't damaged her feet and when her shoes were removed, about 2 days after having them on, she was instantly far more comfortable and returned to soundness over the course of a week. The farrier and I never worked out what caused it, but possibly it was related.
I see Red Horse products use Zinc sulphate for thrush treatment.