Gimp can open .dds files, and at the moment that's the only program I have for 2D stuff.

To elaborate on this, I was referring to the video tutorials (link in OP), where if you look closely the guy has his Porsche door in a seperate polymesh from the rest of the car, and thus it has its own UV map. Not splitting up the model resulted in me getting a "too many polygons" or so error when applying a weight map to the high-res version, and even with 100% opacity, painting the entire model with said weight map resulted in only some square specks appearing, with the rest of the surface unaffected. Splitting up the model solved this, so I'm fairly sure this was the problem.

However, each of my model parts now has its own Texture Projection and UV map, and I don't know how to combine the individual Projections/UV maps into one large projection full of subprojections (which was my approach prior to realising the weight maps don't support that many polys)

The most important question for now is whether I can use textures painted in XSI via layers/weightmaps for sins models (i.e. export them as 2D images and somehow splice them into the layers needed for sins, then save as .dds in Gimp), because if I can't then I needn't bother with getting that to work.

Just how exactly do other people (even the sins devs) produce their textures? I simply can't believe that they print out a UV stamp and paint the entire texture over that, which is what this tutorial seems to suggest.

This, or at least what you're supposed to do between the existing tutorial points, is missing.

Edit: What I mean is while there are texturing tutorials I can't find any explanation of whether these methods work with sins models, and how you're supposed to turn the result from these texturing methods into a set of sins-conform -cl, -nm and -da .dds texture sheets. (for example you link to an "Animated Displacement Maps" video tutorial, which definitely sounds like something sins won't support)

And I know/suspected that the final model must have 1 UV map only, which is my problem as following the tutorial (link in OP) I end up with more than 1, and I don't know how to merge/heal different UV maps from different texture projections together (the alternative being I set up a new projection for the combined model and manually align the UVs with the textures of the parts, which is tedious and inaccurate.

Was the unwrapping done via a built-in tool or manually?

You can have 2 or more different UV maps for 1 model if needed. It involves creating separate materials and then rendering the relevant texture file, UV's and tangents to the separate materials. Check out the Advanced section of the forge tools documentation for a description on how to go about it. When using multiple materials and texture files you will be able to get the resolution your after, but note, there is a high associated memory use when doing this unless you can some how use existing textures.

You can use "unique UV" Texture projection to unwrap in SoftImage.

Mod tool does not export texture clips, but SoftImage will export dds. file format. if you have it.

By the way AutoDesk is now letting people use an Educational version (full version) for a 13 month period if you join the student community, like what they have been doing for 3D max for some time now. Also 25 other AutoDesk products.

If your interested check out;

http://usa.autodesk.com/adsk/servlet/item?siteID=123112&id=14185424

Softimage 2010 is the NOT FREE version of Softimage|XSI mod tool.

Creating a Second Set of Materials is advanced and still uses a single merged object but you create clusters where the different textures will go. I don't recommend this step until you are familiar with the basics.

I think people are exporting to TGA the converting to DDS