[PRoSToTeM@]

Ye, plugin_precache is called from first spawn (from SV_LoadEntities). And usermsg registering is called from SV_ActivateServer (pfnServerActivate callback). SV_ActivateServer is called after SV_LoadEntities. As @Arkshine said the only and best way is to hook FM_RegUserMsg.

Possibly we can add register_message_lazy which would get usermsg name instead of id and retrieve msgid when the first message is sent.