I do believe I've seen something like a cvar, command manager round here but I think they might be a bit much for what you need. Just checking cvars in plugin_init() is better since server load is at it's minimum.
Also AMXX does NOT provide any mechanism of pre-declaring stand-alone cvars (it has something to do with cvar registration and owner plugin). Moving forward, you could edit an always-active amxx plugin and create the cvar there, leaving the rest of the plugins to just access it.
EDIT: Or you could declare the cvar pointer as public and thus fully exposing it to other plugins.