I would suggest using
this admin login plugin because the only way to give access to anything is through admin flags and
Clan Chat.
Without giving someone at least one flag of access, an entirely new plugin would need to be created. There is no real point to doing this. You could just give clan members ADMIN_CHAT access to chat using amx_chat to only admin. Yes they could also use amx_say and amx_csay and tsay but if they're in the clan and you trust them then chat isn't really that big of a deal to give out.