This is a great cut. Perhaps the explicit restriction to only support polymorphic-appropriate apis will help shake out which libs would be good candidates. (no serde, yes ripgrep, etc)
There is a storied history of necessary security considerations for using dynamically linked libraries, the long and short of it is if you load a compromised shared library you're screwed, so.... don't do that. 
The how of "just don't do that" comes down to policy and system design which all lie squarely on the OS vendor and shouldn't really affect Rust's implementation.
I wonder if the abi could be designed in such a way that it would allow libraries to be unloaded and new ones loaded live on the fly, without restarting the process. I guess this is typically not done because a process often entangles itself pretty deeply with its libraries and cleanly disentangling them is not worth the headache so restarting the process when a library changes is common. However, with lifetimes perhaps it's possible to have a process pause, cleanly unload a library, reload a new version, and resume knowing that nothing was missed because use of the old library is tied to its expired lifetime. Maybe with tech like Lt<’a> - lifetimes for fn! being discussed elsewhere right now. This is also kinda similar to how Thesus OS is designed to reload and relink crates/cells at runtime.