Here are some steps to create a Web Part Page using WSS RPC.

Use the Lists WS button to retrieve a list of lists on the current site:

Find a document library and copy the list ID.  Then Click on the WSS RPC button:

Choose "New Web Page" from the template dropdown:

Next, paste the id into the <SetList> element, set the type to "WebPartPage," choose a WebPartPageTemplate, set overwrite to false, and finally, give the WebPartPage a title:

Click "Send WSSRPC" to process the command:

If your results look good, browse to the document library and you should see your new page: