I know nothing about Dynamic Linking or a stable ABI. However I am on the Cargo team. Most of the compile time benefits can more easily be gotten by using one shared target directory for all projects on the same computer. You can experiment with this behavior on stable by setting the CARGO_TARGET_DIR env. There are UI problems with the current system, mostly that there is no good way to clean thing up, without triggering a full rebuild of all projects on the system.
Fairly recent sommery: Tools Team: tell us your sorrows
Long discussion of possible future directions: [Idea] Cargo Global Binary Cache
Place to start contributing to get things moving: https://github.com/rust-lang/cargo/issues/6978