The Great White North, where we get taxed out the wazoo
I would go along with the MRI idea, and pull her shoes. Give her a barefoot break.
A wild and crazy thought also to consider might be she came from Europe nerved in her front feet. Since bute is not allowed a lot end up nerved to fix something we might just give them a little help with. The nerves can grow back, or neuromas can grow and are very painful. Unlikely, but still possible. Just throwing it out there.
If injections of the coffin joint made her sound before and she blocks to the foot why not inject the coffin joint again and see what you get? MRI will tell you about navicular bursa changes, adhesions of the ddft, impar ligament, etc. Did anything show up on the soft tissue phase of the bone scan?