I just finished creating the developers manual to extend Semantic Maps. Like the manual for Maps, it targets developers who want to add support for a mapping service to one of the features Maps and Semantic Maps support. It goes over all the key aspects of the PHP code, like how to add your own service, how to create a query printer or form input class, and what the most notable differences with the parser function classes are. Examples and out-takes are also provided. The manual does not cover the JavaScript aspects of adding your own mapping service, since that code really depends on the API it uses, and although the overall structure might be similar, the code itself most likely is not.
Manual: Extending Semantic Maps
From the archive: written in 2009 and kept for the record. Much of it is out of date.