There is a way to use reverse animation in the *.animation file? Something like <FLOAT>SPEED:-1 or <BOOL>REVERSE:true?

At least in Dusho's Mesh Viewer, it didn't work

Yeah, this file works only in the game / GUTS.

Does anyone know complete list of tags that can be used in this *.animation file? Maybe some hidden features?

I experimented a bit with this, there is a lot of potential. For example, you can make different types of death - cut off a head or a leg.

Well, if a simple method does not work, I'll talk about a more difficult method. A boring method! 

Firstly, we need to convert а *.skeleton file using OgreXmlConverter.

Then, using Notepad++ change timing. For example:
The original animation DIE1.SKELETON.xml

We have the following frames for each bone:

Spoiler (hover to show)

<keyframe time="0">
<keyframe time="0.0666667">
<keyframe time="0.1">
<keyframe time="0.133333">
...
...
<keyframe time="1.56667">
<keyframe time="1.93333">
<keyframe time="2">

Change the name of the animation:
<animation name="Die1" length="2">  -->
<animation name="Envi_alert1_dead" length="2">
and save your file as:
ENVI_ALERT1_DEAD.SKELETON.xml

Now we need to do an automatic replacement. Open both files(original DIE1 and new ENVI_ALERT1_DEAD) for easy operation (copy / paste).

Note: We must act carefully - it is better to use a string replacement.

Step1.
The first automatic all-replacement:

<keyframe time=" --> <keyframe time="9

All our frames have changed:

<keyframe time="0"> --> <keyframe time="90">
<keyframe time="0.0666667"> --> <keyframe time="90.0666667">
...
...
<keyframe time="1.93333"> --> <keyframe time="91.93333">
<keyframe time="2"> --> <keyframe time="92">

This is necessary to avoid errors in a future replacement. 

Step2.
Replace timings Conversely, step by step:
<keyframe time="90"> --> <keyframe time="2">
<keyframe time="90.0666667"> --> <keyframe time="1.93333">
...
...
<keyframe time="91.93333"> --> <keyframe time="0.0666667">
<keyframe time="92"> --> <keyframe time="0">

If done correctly, you should get the following:

Spoiler (hover to show)

<keyframe time="2">
<keyframe time="1.93333">
<keyframe time="1.56667">
...
...
<keyframe time="0.133333">
<keyframe time="0.1">
<keyframe time="0.0666667">
<keyframe time="0">

Save your file and convert it using OgreXmlConverter.
If you've done everything correctly, you should get a new reverse animation ENVI_ALERT1_DEAD.SKELETON and now you understand why I was looking for an easier way. 

Darn! Now you have got me thinking this method could also be used to take only certain keyframes from an animation, to be used as a new animation.

It is possible, I have used it before for my new animations (Gatling, Peacemaker).

Does the keyframe timing somehow correspond to the "frames" counter in OGRE mesh viewer?

Yes, but not directly. Animation contains key points - "keyframes" - these points given path of each bone over time. 
Animation is connected with the frame rate - 30fps. Your animation with a length 2sec can have 2 keyframes, but there are 60 frames in OGRE mesh viewer.

Haha!  I've done it! I made 3 new animations from the DAG_DW3.SKELETON file!

It took a lot of trial-and-error to find the right keyframes on which to start and stop each animation, but I finally got it

SPECIAL_INVINCIBILITY_START - uses the first one-third of DAG_DW3

SPECIAL_INVINCIBILITY_LOOP - well, it's just a steady 1-frame animation.  I first tried to go for a "vibrating/pulsing" effect, but couldn't get it to look right.  So I abandoned that.

SPECIAL_INVINCIBILITY_END - uses the last two-thirds of DAG_DW3

Now to get it into the skill so I can see if it looks right...

Thanks Kva! Yes its not difficult, just tedious because it has to be done track by track.  But I also learned that the keyframes for all tracks have the same time-stamp.  So when youve found the right keyframe for one track, its the same for all   Also makes it easier for the renumbering.

Thanks too for the labeling tip!  Thats a neat trick I'll use it the next time i do something like this.