It's very unfortunate that Spindle has stalled, short of adding Tapestry 4 support. Fortunately, it's a big world and others have started stepping in to fill the gaps. On approach is Cognition, which uses a visual builder tied to a data model driven architecture.
Another one just brought to my attention is eZing Builder. From what I can tell (the site's a little short on documentation, and SourceForge is currently down for maintenance), it's a general purpose WYSIWYG editor for web applications, with specific Tapestry extensions. Built on Eclipse 3.2, and open source. Looks nice.