Real TCO works for arbitrary calls in tail position, not just self calls, so that code like the following does not cause a stack overflow:
(letfn [(e? [x] (or (zero? x) (o? (dec x))))
(o? [x] (e? (dec x)))]
(e? 10))
Clearly you'd need JVM support for this, since programs running on the JVM cannot manipulate the call stack. (Unless you were willing to establish your own calling convention and impose the associated overhead on function calls; Clojure aims to use regular JVM method calls.)
As for eliminating self calls in tail position, that's a simpler problem which can be solved as long as the entire function body gets compiled to a single JVM method. That is a limiting promise to make, however. Besides, recur
is fairly well liked for its explicitness.