Is this a production situation or a one off?
I've never seen any commercial offering of endmills that will do this. I can well imagine the extreme delicacy of the tip of such an endmill, resulting in a short useful life for the tool.
I would side mill it. For a single job, grind up a flycutter and make a little arbor to hold it in a regular endmill holder. However, grinding the tool requires a sample spline to test the tool shape. If you have a male spline sample, you can cast an inverse off of it with a bit of plaster of Paris, then you've got a gauge to grind and fit the tool to.
For production, you could likely get a tool grinding shop to custom make the tool, using something like a woodruff cutter as a basis to begin from. A small woodruff cutter might get you within 3/8" of a shoulder. If you must get closer to the shoulder than that, you'd almost be looking at some sort of EDM procedure.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)