Ok, I've now got this working. writemime it seems is not used by show (or at least not for me), only by display.
This use of a custom Display, is slightly more protracted, but here's <https://github.com/samuelcolvin/Bokeh.jl/blob/ca091f0f7941307e82ddf58ce920ba71bae928eb/src/display.jl> an example.
