<lang-hat>Not speaking for the team here, but the bar for a new keyword is generally extremely high, even one that looks like a macro. I suspect that "there are two macros (or two macro forms)" here would be considered a better solution than adding a keyword</lang-hat>
Note that Rust generally tries to avoid life before main, so I suspect "a new block that is at the start of main()" wouldn't happen. (Among other reasons, because what would that mean for something compiled as a library and might be used somewhere without a Rust main.)
That said, if we had something like Idea: global static variables extendable at compile-time (which has a chance, if someone's interested in driving it) could do the "extensible collection of bits of code" that could be called manually from entry-points. Which I agree would be a nice use of this, and mean that the assertion macros here would just be macros.