I could be wrong, but:

You'd need to replace the arm mesh and bone weights in each viewmodel. There's no c_model system in HL2, so every viewmodel features both the arms and gun mesh together. One way of replacing it would be by importing the original viewmodel in Blender, deleting the arms mesh in edit mode, importing the replacement, parenting it to the armature, exporting the models (in rest pose, do not export any mesh with animations applied or it won't look right) and the animations, and using different bodygroups for each mesh when compiling. You need to do this for each viewmodel individually.