Ah, in fact, there is a solution! DirectX WARP (basically, 100% software DirectX). Of course this is slow as Hell, not usable in production, but enough for my tests purpose for now.
You need latest DX11. Then, run dxcpl (e.g. from the command field of the start windows menu), add UnrealEngineLauncher.exe to the list, and enable ‘Force WARP’ option.
Once downloaded, you’ll have to add UE4Editor.exe to that list, too (and probably other executables, if you need them).
Note you can also force dx debug state from there… Just a bit sad there does not seems to be any ‘profile’ system, all apps listed get the same settings it seems.