Thanks Alistair for the MR; I can't seem to find the time for getting started with this endeavor myself.
If I recall correctly from the last time I had a conversation with Alexandre about this, the plan was to factor out common code as needed for reuse, but have it live in one of the existing DLLs for the time being (it could either stay in d3dx10 or be moved to d3dx11, not sure - I haven't thought it through) and use PARENTSRC for sharing. Once we've gone through most of the d3dx1* functions and the "helpers API" is stabilized we could then split all that stuff out to a separate DLL and get the additional benefits of reduced total DLL size and compilation time.
I'll try to prepare an initial MR following this new plan. No guarantees that I'll get to it any time soon.