Yes it's possible, and will have the ambiguous behavior in the case of pread as you describe. I haven't thought much about whether I want it to be possible or not; I guess not?

If you have an idea how to eliminate ambiguity that is backwards compatible, I'd be interested, but I'm not sure whether it is worth a large time investment, since I don't think this issue comes up for most people (why do they want two different contexts, and if so, they should just use the _with methods to disambiguate it)

I'm not sure if there's a way to do it with backwards compatibility, but I think you'd accomplish only being able to implement once by having the context type be an associated type instead of a generic parameter. Whether you can create a backwards-compatible wrapper or not is another question.

Maybe put it on a "if I'm already going to make a breaking change, think about making a breaking change here too" list?

I think it makes sense for it to be possible. It's a nice elegant way to describe how to read the same type in different ways:

let str1 = buf.gread_with::<&str>(offset, LengthPrefixed);
let str2 = buf.gread_with::<&str>(offset, FixedLength(123));

// LengthPrefixed and FixedLength definitions left as an exercise for the reader