Macros would need to specify whether any expressions will be evaluated in tail position. However, expressions in macros don't look like they should be subject to TCO, so the default assumption should be that they aren't unless declared otherwise. Do you have any examples of cases that would be likely to cause confusion—in particular where a function call appears to occur in tail position in the code but can't be TCO'd because of a macro?
For example I see sometimes macros which generate code with compilation quality (speed, ...) declarations for all or parts of their code. Depending on the combination of qualities TCO might be enabled or disabled in code sections.
If TCO is guaranteed at the language level, as in Scheme, then it will always be enabled regardless of compilation settings. Debug builds are no more tolerant of stack space leaks than release builds. The fact that TCO isn't guaranteed is the problem here.
Your point? The page you linked to specifically says that Kawa only implements a subset of modern Scheme—by which I mean R5RS or later. Early versions of the Scheme language standard didn't require TCO, but all the recent ones do. This doesn't affect the core point that if TCO is guaranteed by the language standard, as in modern Scheme, then it cannot be selectively disabled because doing so would break perfectly compliant code.
