The first step towards getting a test-case is to isolate all std.modules.

For one follow the imports and copy files to a side, re-name the pacakge to e.g. stdx and adjust imports.
By the end you should have a stand alone bunch of files inside of stdx folder then 
running 
rdmd --main -unittest std/format.d
and will trigger a crash.

The rest of the task could be left to dustmite. https://github.com/CyberShadow/DustMite
Just make sure you've read the instructions carefully and specified the correct failure condition (e.g. via grep of rdmd's output).

> Is this worth creating an entry, or is the problem too specific?

The problem is unexpected failure at CTFE and these thing are bound to pop up later on. We'd better find it now.