Alloys/metallurgy : promising elements for for Sn-based solders
Lithium might be tricky to alloy in an oxidizing atmosphere (e.g. air), and Pt might take some effort to form an alloy, because of its high m.p., which, however, is still less than than the boiling points of Bi or Sn. Other than that, there should be little trouble forming alloys. 

You can search on line for "phase diagram alloy components" to find the melting point for at least some of the alloys. E.G. an online search for

phase diagram "copper-indium" alloy


turned up this link at Springer-Verlag GmbH, with the diagram below.



That said, I would expect the "10-20 % copper" requirement precludes melting point less than 300 C, even with low-melting In, Hg, Ga etc.

BTW, I would not use "dopant" for a component of an alloy that will not appreciably change its electronic properties; it is just another component, be it minor, of a ternary, quaternary etc. alloy.