Maybe I'm completely missing your problem, but..
Code:
register_cvar("a_cvar", "1");
new cvar_pointer = get_cvar_pointer("a_cvar");
register_cvar()
already checks to see if the cvar has been declared previously. Whether this is better is debatable because if the cvar does not already exist then you've just discarded the return value which you will then be interested in.
If these plugins are only for personal use then why not register the cvar in admincmd.sma or one of the other base add-ons..? In fact, why is this such an issue in the first place? plugin_init() should be the last place you look at optimising. The rest of the code should be perfect before you even consider this.