The TCO sounds like an ugly hack to me, but I'll take it. How would this interact with the magical AST merging? I would assume that languages with different calling conventions can't just merge like that.
Let's say you're in a particular function at a tail call node. Since the tail call node is semantically (from the point of view of the Java program) many function calls deep into the implemented language's function and we want to get back to the start, we want some way of exiting a whole stack of function calls in one jump.
Exceptions offer a simple and neat way to do this. You throw from the call site and catch back at the start of the function. Graal will inline all of the internal function calls up to this point, since Graal uses inlining extremely heavily. This means that Graal will see the throw site and the call site next to each other in one block of code, and consider it to just be a jump.
Because Graal is just inlining Java functions, and the actual target language's call stack is stored separately as a particular object, the target language's calling convention for a large part doesn't matter. The jump to the first node doesn't invalidate any of the target language's stack - that has to be done by the language implementor manually at that point.