Chris O’Brien’s wrote a great article about, customizing the _layouts/settings.aspx page with CustomActions

http://www.sharepointnutsandbolts.com/2007/07/using-customaction-to-modify-system.html

For example

<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <CustomActionGroup
   Id="CustomActionGroupAdminSetting"
   Description=""
   Title="$Resources:Resources, CustomActionGroupTitle"
   Location="Microsoft.SharePoint.SiteSettings"
   Sequence="9" />
  <CustomAction
    GroupId="CustomActionAdminSetting"
    Title="$Resources:Resources,CustomActionTitle"
    Description="$Resources:Resources,CustomActionDescription"
    Location="Microsoft.SharePoint.SiteSettings"
    Sequence="11"

    Id="CustomActionSettings"
    >
    <UrlAction Url="/CustomSettings.aspx"/>
  </CustomAction>
</Elements>