Write an RFC, is my opinion
I realized that my wording was not clear - I only intend to upstream support for 16-bit pointers (the build system modifications, however useful, are only half-baked right now).
The patch I hope to get merge consists solely of copy-pasting constants:
pub const DTOR_NEEDED_U16: u16 = repeat_u8_as_u16!(DTOR_NEEDED); // added this
pub const DTOR_NEEDED_U32: u32 = repeat_u8_as_u32!(DTOR_NEEDED);
and match arms to handle 16-bit pointers
match target_pointer_width {
"16" => TyInt(TyI16), // added this
"32" => TyInt(TyI32),
...
}
The Rust RFC README states that things adding solely "better platform coverage" and "additions only likely to be noticed by other developers-of-rust, invisible to users-of-rust" do not require an RFC.
That said, I am happy to write an RFC if it would be preferred 
This post is mostly wondering whether the compiler team thinks that adding support for something that is not even going to be used in-tree is going to be worth the (admittedly small) maintence burden.