Flip normals without destroying custom normals?

Hi,
I am trying to do the following. I import an FBX with several meshes, all with custom normals. Some of the meshes have normals pointing the wrong way. How can I flip the normals (ie. rotate exactly 180°, or scale with -1), without destroying the very valuable custom normals orientation?

If I use the CTRL-N command, or if i click on “Flip Normals” under “Tools–> Shading/UVs”, then Blender not only flips the normals, but recalculates them (based on the face orientations), leading to ugly shading artifacts.

Thanks!

Well, noone seems to have had an answer.
I’ve thank God found it myself, so if anyone is curious: there is an addon (bundled with v2.79) called “Custom Normals Tools”, you have to enable it under user preferences. This then adds a category to the Tools Tab of the toolshelf called Custom Normal Tools, and this has exactly ONE button, called “Flip Custom Normals”. This works in object mode and flips the normals without wrecking their orientation. Exactly what I was looking for. Yay!
(Although it’s called “Tools”, the addon doesn’t seem to do anything else.)

Thanks for posting back with your solution. Its useful to know that the Custom Normals Tools can handle this.