Well, I kept hunting around and found this seemingly easy solution, and it works for me.
in the aspx page, in the control, I have
ServiceMethod="GetTagNames"
and in the cs page I added this in the page load:
[System.Web.Services.WebMethod]
public static string[] GetTagNames(string prefixText, int count)
{
mydatabase db = new mydatabase();
return db.TagTables.Where(n => n.TagName.StartsWith(prefixText)).OrderBy(n => n.TagName).Select(n => n.TagName).Distinct().Take(count).ToArray();
}
Hopefully it will help someone else out there!