Although unicode-math is supposed to be comprehensive, users should be able to add their own symbols. Perhaps this could be investigated via the TU encoding support in fontspec.
Bruno's suggestion (via #371) was:
Also, it would be nice if there were a command \DeclareUnicodeMathSymbol available for the user to use in a .tex file, similar to fontspec's \DeclareUnicodeSymbol from tuenc.def.
Although
unicode-mathis supposed to be comprehensive, users should be able to add their own symbols. Perhaps this could be investigated via the TU encoding support infontspec.Bruno's suggestion (via #371) was: