Blender 3d is one tuff nut to crack.

[BNG Da BZ Fool]

I know it's more work to create an XSI importer, but there isn't really a single decent application to work with existing XSI files and this is especially true for working with the data pack XSI files from BZSCRAP.COM. I have to convert any I'd like to use to another format like COB or X or 3DS. The animated files never translate correctly and end up with non animated XSI conversions. Also from what I've seen with your XSI variant there's really no viable way to examine the files short of within the BZII map editor.

It's not really all that big of a deal, I guess any changes could be done to the Blend file and then just overwrite the XSI file again to make changes as needed.   

[anomaly]

On that note, OM, are there any import settings in Blender, specifically for .x?  I'm able to import stock .xsi models into Blender using the .x importer but, all frames are renamed to default names and the order of hierarchy is completely eliminated. Is there a way to prevent that from happening?

[OvermindDL1]

Not a clue, I deal with the OgreMesh format, and BZ2's .mesh format, I do not really like the others due to a lack of solidarity.
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[anomaly]

Ugh.  My kingdom for a .xsi importer/exporter.  Seriously, I don't think this stuff could possibly be more complicated. 

BNG, didn't you find a way to import .xsi into GSL?  I can't get it to work right, all I get when importing is a flat plane made of a tangle of polygons.

[BNG Da BZ Fool]

A couple of months late and a dollar short, but there's no way to directly import XSI files into GSL or TS 3.2. Alternate ways would be to use 3DEX to batch convert the data pack XSI's to another format like 3DS, COB, OBJ, LWO, etc.

Don't convert to X as GSL can save to an X version that Threed seems to love. Another drawback to GSL is that models of more then 650 faces can't be saved again. However, one workaround wold be to edit the high poly model down by reducing the poly count by selectively using the delete vertices's tool in the edit menu of GSL.

From what I've noticed all of the data pack files are triangulated which effectively doubles the poly count. Right clicking an object in GSL automatically enters into the edit menu. Also, after reducing the poly count you can open the X save options prior to saving the model as X and uncheck the option to triangulate the model.

I've used the object library feature in GSL to dissect and save BZII models as separated objects after creating a new folder named after the model itself. Then in the scene editor selected each part individually adding them to the new folder allowing me to build vast object libraries of all the BZII models.

Another, side benefit with GSL is the key frame editor in GSL easily allows me to create animation sequences, save them as X files, then convert them to animated XSI models, and with a little notepad editing magic of cut and paste;I've already produced several crudely animated models. I still don't really know what the hell I'm doing, but eventually my goal is to get animations working in the game...BNG.