Solaris has had a notion called an "agent thread" for a long time. Basically a debugger can attach to a process and start an "agent thread" in the victim proce

Solaris has had a notion called an "agent thread" for a long time. Basically a debugger can attach to a process and start an "agent thread" in the victim process to run text that is sent to the agent thread to run. This has been used for all sorts of magical features like redirecting a running process' standard I/O handles. This is sufficiently hard to use that it was roundly hated. What you really want is a whole, parallel linker-loader environment in the agent thread so you could say "load this shared object and call this function in it with these arguments".