Hello There,
This is an interesting idea, however, I just wanted to share a few thoughts from an implementation (impl.) perspective. Happy for you to use some (or all) of this advice (with us or another provider :) ):
1. An elaborate dev plan will be key to the success of this piece so that the actual (hidden & visible) deliverables are agreed upon prior. This is to ensure that the dev process is executed suitably to avoid performance &/or compatibility issues moving forward.
2. An (optimum) impl. of this design will require that the page is not made content heavy and that only appropriate elements are used to reach the desired result. For instance, instead of using images all through to reach the desired visual result, some (optimised) solutions could include the use of HTML5 Canvas, SVG and advanced CSS clip-path technologies.
3. Each of these solutions will need to be compatibility-tested with tech. that your target audience is currently or will be using. This includes responsiveness/small-screen compatibility.
4. From a pure Ux perspective, we also suggest an alternative menu in order to cater to the audience who may not be comfortable with this non-traditional approach. For instance, an alt. off-screen menu may be one solution.
We're a Melbourne based agency specialised in Ux, performance & usability. The quote covers all of the above (as well as impl. in WP). We're new to Freelancer but have been around for 10yrs. Let's catch up for a coffee to get this rolling :)