Custom HUDs use assets that are included with Tremulous that were intended for use as HUDs, anyway. A lot of HUDs are just rearranged versions of the default. If you can find, somewhere, an alternate skin for anything that is packed for Tremulous, and was intended for use, then I'm sure you'd be able to use that, but otherwise, the server either has to have the mod, or be running on sv_pure 0.