Sloping top tube?

I can see where the frame builder is coming from.

Compact geometry equals a really small/stiff rear triangle. Ride quality is still ok because the seatpost is relatively long. The plan set out in the OP is the opposite and would result in a really short seatpost, and maybe a pretty crap ride, with none of the aerodynamic benefits that lo-pros are all about.

I think the frame builder may have refused to build it because they can't see a good reason to.

It'll look good though!

+1

Frame builder has probably refused to build/said it's not possible as they do not want to have their name associated with a frame that has strange geometry that a lot of people will fail to see the point in and if anyone rides it they will think that the ride quality is pretty poor because of the shorter seatpost etc and presume this is the builders fault rather than realise that the frame has been built to specific requests.