Hi guys, I know this is a dated post, but i wanted to update it. While Extension Points was removed from BI4, it was re-introduced in BI4.1.
We have been using Extension Points to introduce mapping capabilities as a part of our CMaps Analytics product.
Extension points now is a little different than the 3.1 days because extensions are loaded in the left side pane instead of the report canvas.
I did want to note for this thread that the use of Google Maps in Webi without a Google enterprise license would violate their terms of use for their free APIs. Something to take a look at before moving anything into a production environment so you don't put your customers at legal risk.
If you are going to build your own maps, there are some cheaper open source alternatives now than are just as good as Google these days for interactive mapping to keep you safe from licensing issues.
If you need a an extension and mapping for BI4.1 with enterprise SLA and support, give CMaps Analytics a go: http://cmapsanalytics.com/webi.html
If you guys have any questions about extension points for BI4.1, let me know.