Added a new custom field to a document library, wanted to update the metadata of
the custom field with something like Name = Anthony using Front Page RPC method
Was not able to do it, tried with 
1. '\='
2.'%3D'
3. '='

The Value to be used is %5c%3d
For setting the metadata value to Name = Anthony -- Name %5c%3d Anthony.

Code sample to set the metadata information of a custom field using Front page
RPC

public void SetMetaData(string uri)
{
Uri myUri = new Uri(uri); // URL of the doc like
http://<servername>/sites/<siteName>/Shared Documents/<document name>.doc
string webUrl // sites/<Site Name>
string fileUrl; // Shared Documents/<document Name>
UrlToWebUrl(uri, out webUrl, out fileUrl);
string postBody = String.Format("method=set+document+meta-info&service_name=" +
webUrl + "&document_name={0}&meta_info=[Hello;SR|Test+'&#61;'+Dat]", fileUrl);
SendRequest(myUri.GetLeftPart(UriPartial.Authority) + webUrl +
"/_vti_bin/_vti_aut/author.dll", postBody);
}