In the past i posted a script i wrote that attempted to copyweights between two identical topology meshes but only on specified vertices (not all the mesh). This tool was rather hard to use in practice requiring lots of parameters (na_copyWeight)... e.t.c.
Today i found that Maya's builtin Copy Weights tool also can copy weight using a subset of the vertices of the mesh. It worked great and I was able to use it in practice to merge two separate skinned meshes for the lip and head.
To use Maya copy weights tool for copying subset of vertex weights:
-- assuming your source and destination mesh have same influences and topology
-- first select the vertices on the destination mesh for which we want to effect
--- then press command and also select the source, then the destination
---then in Maya's copy weights tool use copy by name
Hope this was helpful,