That's not...totally accurate paper. Go over at OldUnreal and look at the swamp titan: https://www.oldunreal.com/cgi-bin/yabb2 ... 1345670574
That was done to the base titan model. I pestered Q to find out the process but he never wrote a tutorial on how he did it. At least we know it's possible to do.
Actually... what Paper said is quite accurate indeed.
What you have seen is already described in the first post text:
This is a package which includes Unreal vanilla pawns edited in some extent (I mean, meshes mostly but other things as well)
meaning that there's really no science to it: he just exported the models, opened them in a 3D modeling program, edited them (model and animations), and imported them back to Unreal.
It has nothing to do with attaching stuff to vertices.
Having that said, Unreal 227 does have functions to be able to attach stuff to the mesh vertices, exactly with the intent the OP described: to wear hats, armor, that sort of thing.
However, UT99 doesn't have any of such, so it's quite impossible to attach stuff to vertices as is.
But a native extension could be made to add this functionality, as at the native level you have access to this stuff (I experimented with something similar myself years ago when I was making my first steps into the native code), but native extensions are relatively hard to maintain and distribute reliably.
But one thing I personally intend to do in the future, is to map the existing vertex locations of each player mesh for each animation in a separate structure, and use this map to "attach" stuff to those vertices, where based on the mesh used, scale, etc, along with the frame being played, I could then tell where each vertex would be. But this would be specific maps generated from specific meshes, with potentially an external tool yet to be written, and not a general purpose solution of course.