Isn't it as simple as adding a ptrace(PT_COREDUMP,...) and calling
   coredump() in it?


as i recall when i looked at adding gcore a few years back, it turned out
that one needs to swap contexts to the process we are trying to attach
for coredump() to work, and there's no support in the kernel to facilitate
that.  thorpej had some almost complete implementation to allow this but
he never finished it... i think he got the idea for sunos4.?