I think implementing a second Rust compiler in a different language for the purposes of a DDC-style comparison wouldn't be too hard: it could assume that the input source is type/borrow-correct and so wouldn't have to do any semantic checking, "just" type inference and code generation. Of course, this is still non-trivial, but probably easier than an implementation of the borrow checker in another language (although the latter would be nice too).
(All that said, I'm not personally going to put my money where my mouth is, haha.)
In any case, I'm not sure one can properly trust any product of a
rustc compiler in this context, even cross-compiled versions/early stages: a hypothetical back door could have touched any of them.