Molecular mechanics force fields for metals?
The modelling of metals using a 'simple' potential is relatively difficult. I have a suggestion for a force field, though it does not fit exactly all your criteria, but I would like to offer it anyway.

Adri van Duin has developed a force field called ReaxFF. This force field is non-proprietary and is readily available in the open-source package LAMMPS. This force field gives pretty good results and can be applied to any kind of elements, it can describe chemical reactions and is also valid for hetero-atomic interactions. That being said, the biggest challenge when using the ReaxFF force field is that you have to fit / train it to your system. This requires, for instance, a database of ab initio calculations that are representative for your system.

If you cannot find any force field, than this might be a interesting alternative to developing a force field completely from scratch.