Yeah, a gui.set() might be useful for the kind of example you mentioned. If you find that you need to do these kind of things very frequently you could implement it yourself as a helper function and either monkeypatch the gui namespace or put it in a Lua module.
yeah, it is kind of what im doing now: i have a module which adds to gui functions set_x, set_y, set_alpha , holds local for module vmath.vector3 and vmath.vector4 to avoid creating temporaries for each call. I guess other properties need to be modified significantly less frequently and it is fine for now, but it seems to me usage of gui.set would be clearer. I hope you will be able to find time for this.