For one, I don't think this would be a big deal at all from a gameplay perspective.
Technically, this wouldn't really be that bad; it's more a matter of enabling functionality that's already there, since cg_thirdPerson is existent but disabled. However, I don't know much about the sv_pure checks, but I believe such a client modification wouldn't pass them. Thus, approval would have to be given from developers to put the implementation in both server and client sides.
For now, you'll have to settle for the fact that you can do it in demos.