Rust could target it, but not as easily as you might think. LLVM IR is not portable (for example, here's some of the code necessary to get the calling conventions right in the LLVM IR for x86-64 [1]). The runtime and scheduler would have to be ported to the Pepper API if you wanted stuff like GC, I/O, and threads (which most Rust programs will want).

[1]: https://github.com/mozilla/rust/blob/master/src/librustc/mid...