You could give your friends that you want to be on the "whitelist" for your server a client mod that packages GameConnection::setConnectArgs()
I recently did this for my Underground mod GUI check, thanks to Zack0Wack0 and Trigun:
function GameConnection::setConnectArgs(%a,%b,%c,%d,%e,%f,%g,%h,%i)
{
%i = setField(%i,getFieldCount(%i),"Underground" SPC $UndergroundGUI::GUIVersion);
Parent::setConnectArgs(%a,%b,%c,%d,%e,%f,%g,%h,%i);
}