Art - I can confirm the following:
I made a basic box and assigned a die texture to it. (one to six dots) - dieface.bmp
I made a night texture (the basic texture, added a layer, filled it with a reddish color and some "lights". and assigned it the _LM suffix
In FSDS, I assigned the texture, flagging it with the night texture tag, so that it does come out with the ,N tag in the dialog.
From FSDS, I compiled (to my local business airport - KORL Orlando Executive) to the Addon Scenery/Scenery folder.
I copied the two textures - dieface.bmp and dieface_LM.bmp - to the addo Scenery/Textures folder.
Since Paint SHop Pro saved the bitmaps as 24-bit images, which FS abhors, I used imagetool to change it to DXT3 format (just because I could).
I opened up FS-X, went to KORL and I do see the night texture at night , and the day textures in the day.
Sorry it's taken so long to write my reply, but I'm not a scenery modeller so for me, this was a bit of a challenge to figure out for myself (with Farfly's post as a starter, that is).
I'm hoping that a gmaxer (ARno?) can chime in and say how is it done with gmax/Max ....
I'm sending Adam my box, textures and screenshots so he can check it out and put it up on the blog.
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I've answered my own question:
http://www.fsdeveloper.com/forum/showthread.php?t=11334&highlight=night+textures
Arno indicated basically the same thing FSDS does.