What user context do SharePoint timer jobs run under

I'm not at work so this is off the top of my head, but: If you get a reference to the Site, can you try to create a new SPSite with the SYSTEM-UserToken?

SPUserToken sut = thisSite.RootWeb.AllUsers["SHAREPOINT\SYSTEM"].UserToken;

using (SPSite syssite = new SPSite(thisSite.Url,sut)
{
    // Do what you have to do
}

I recommend always using domain accounts - SharePoint works best on servers connected to an Active Directory server. For production environments a best practice is using a least privilege account. I always create the following domain account dedicated to SharePoint services:

DOM\spservice

You do not need to grant any special privileges to this account as SharePoint will automatically do this for you when you specify the account during setup.