There used to be a hack before the AccountManager was reased, I started a thread about a year ago on the android developer group, but it has been removed. There was an undocumented method that you had to access through reflection. I can't seem to find it anywhere now, like google has deleted the thread or something. I found something similar below, but it's not the one I had working.
http://donpark.org/blog/2009/01/24/android-client-side-oauth
At worst case, most devices that are out now, should eventual get 2.1. So you could just make them login then validate and get the auth key from google, and if they are on 2.1 use the AccountManager and don't bother them with the credentials. something like below
WebRequest req = HttpWebRequest.Create(
@"https://www.google.com/accounts/ClientLogin? accountType=GOOGLE&[email protected]&Passwd=pass&service=gbase&source=sadboy");
WebResponse resp = req.GetResponse();
string all;
using (StreamReader sr = new System.IO.StreamReader(resp.GetResponseStream()))
all = sr.ReadToEnd().Trim();
int auth = all.IndexOf("auth=");
string auth = all.Substring(auth, all.Length - auth);
http://developer.android.com/intl/zh-TW/resources/dashboard/platform-versions.html