A followup on this, for anyone that's interested: if you select Project / Project Tree / Edit File Types & Categories and add .s and .S to the extensions for Sources then the assembler files will be treated as source files. This works because gcc will, when presented with assembler source, handle it properly (presumably by invoking the appropriate as executable).
Only problem now is that the editor doesn't open it because 'no handler is registered for this type of file'. Any suggestions on how to tell the editor that a .s file is OK to open, even just as text?
Colin MacDonald