In Europe, there are atmospheric exposure levels set for all hardwood sawdust, as it is recognised as a carcinogen.
At the time given, it would have been impossible to make a piano without lead. All pianos used lead in the action. There are little slugs of lead in the keys, hammers and the actuators.
It is vaguely possible that the company prepared its own nitrocellulose varnishes. This could have left very toxic and carcinogenic residues. Also, nitric acid was sometimes used to color wood, and can likewise leave residues.
If the wood was treated in any way, it would most likely have been painted with a solution containing mercury to protect against mold and worm.
While the heavy metals may concern people, I would say that putting a building on top should stop them leaching away into adjoining areas, and is better than growing poisonous cabbages on the site. The toxic and carcinogenic nitrous residues would have little effect through a concrete raft. As long as the contractor is aware that there may be dangers to his workers, building something on the site would appear to be the best thing to do, as the site will be cleared of the bad things. If the site is dangerous at present, it probably represents a risk to children - this risk is lessened when laid under concrete.
I know exactly what it is like to live near, and drive around, a development that will do little to improve my neighborhood, while impairing the short-term quality of life for those near it. While I can sympathise with your situation, I think you might be better off objecting on other grounds, such as impaired traffic flow or loss of parking due to the projected construction activity.