I remember seeing this being done in some mod, could it have been Azure Sheep?
If I recall correctly, there was basically a duplicated set of weapons, one that used the uniform-clothed hands and another with the hands wearing the HEV suit. Then at some point in the campaign you would be stripped of your weapons and find yourself a HEV suit before being given the HEV-variants of the weapons.
This is of course a game code change, not something that can be done purely through mapping.