The UDOO X86 https://www.udoo.org/udoo-x86/ is a single-board computer with an Atom/Celeron/Pentium and Arduino-compatible headers.

(On the minus side, though, the Arduino-layout headers are connected to an Intel Curie, which at this point is now orphaned. The Curie is attached to the main CPU via USB.)

In a similar vein: http://www.86duino.com/?page_id=11

Can you still buy 8086's or is he filching these from old PC hardware?

Yes, you can still buy them:

https://www.mouser.com/Search/Refine.aspx?Keyword=CP80C86

Intel doesn't make them anymore, but Intersil does (and the '286 too), and is mainly targeting the military/aerospace market.

However a used/NOS one will be cheaper.

This seems timely with this other recent post:

https://news.ycombinator.com/item?id=16938029

Clever. Sadly not a lot of detail in the article about how it was designed, or how it works, or.. well... anything.

Shame - feels like there's a lot they could say about it.

Look at the links on the page; this, for example, provides a schematic among other things:

http://tech.mattmillman.com/projects/8od/hardware/