Sorry this is a bit old - I don't use this kind of thing myself so much.  Drop me a line for what you want to see in this, or help continuing development.
As it is, there are two parts to this:
- A client-side GUI for creating XProc files, implemented mainly as a configuration for the great WireIt library. 
  (there's a more recent version of this on the gh-pages branch).  The WireIt configuration was generated by applying an XSLT file to transform the official XProc language description.
- A servlet wrapping the Calabash XProc processor.

Running implementation of this up at http://feedscape.appspot.com  -- very preliminary.
Steps2Wires.xsl  -- run 'saxon -o:xproc-language.js typed-library.xml Steps2Wires.xsl' to generate xproc-language.js
add_types2lib.xq  -- run 'saxon-xquery' on this to generate a typed version of the standard step library from the W3C XML document.
index.html
typed-library.xml
types.list  -- set of all the types used in the options.
types.xq
xproc-language.js