The AllUsersWebPart tag allow us to insert one webpart and place it where we want using WebPartOrder and WebPartZone attributes. The content of the tag is the dwp (or webpart) of your webpart. For example, for a dwp:
<AllUsersWebPart WebPartZoneID="MiddleLeftZone" WebPartOrder="0">
<![CDATA[
<WebPart xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://schemas.microsoft.com/WebPart/v2">
<FrameType>None</FrameType>
<AllowMinimize>true</AllowMinimize>
<IsVisible>true</IsVisible>
<Assembly>Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c</Assembly>
<TypeName>Microsoft.SharePoint.WebPartPages.ContentEditorWebPart</TypeName>
<ContentLink xmlns="http://schemas.microsoft.com/WebPart/v2/ContentEditor" />
<Content xmlns="http://schemas.microsoft.com/WebPart/v2/ContentEditor">
<Value><iframe src="$Resources:Summary_Url;" frameborder="0" scrolling="no" width="100%" height="100%"></iframe></Value>
</Content>
<PartStorage xmlns="http://schemas.microsoft.com/WebPart/v2/ContentEditor" />
</WebPart>
]]>
</AllUsersWebPart>
For a webpart:
<AllUsersWebPart WebPartZoneID="MiddleLeftZone" WebPartOrder="1">
<![CDATA[
<webParts>
<webPart xmlns="http://schemas.microsoft.com/WebPart/v3">
<metaData>
<type name="MyClass, MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=f3b9d1137704f880" />
<importErrorMessage>No se puede importar este elemento Web.</importErrorMessage>
</metaData>
<data>
<properties>
<property name="AllowClose" type="bool">False</property>
<property name="AllowMinimize" type="bool">False</property>
<property name="AllowConnect" type="bool">True</property>
<property name="ChromeType" type="chrometype">None</property>
<property name="Hidden" type="bool">False</property>
<property name="AllowEdit" type="bool">False</property>
<property name="AllowZoneChange" type="bool">False</property>
<property name="MissingAssembly" type="string">No se puede importar este elemento Web.</property>
<property name="ChromeState" type="chromestate">Normal</property>
</properties>
</data>
</webPart>
</webParts>
]]>
</AllUsersWebPart>
I haven't tried before but I think it can be done once the web has been created. Try it and tell us.