Here's the (recently updated) process for adding things: Propose a change to the language - The Rust Language Design Team
To have a chance to stabilize, it would need a plan to stabilize on all supported architectures. IIRC last it came up there was no native tail call support on WASM, for example. So, in general, it might not be possible to enable arbitrary tail calls.
The question, then, is what kind of more-restricted construct could meet your needs? Is intra-crate calls enough? rustc could, hypothetically, always rewrite such calls into a loop itself.
Or maybe it doesn't need even that much. The most feasible version might be an inside-a-function custom control flow construct, like described in [Pre-RFC] Safe goto with value - #9 by scottmcm. Then, instead of become dispatch(…) it might be continue 'dispatch, locally in the function so there isn't even tail call optimization needed since it'd just be one big CFG.