With TWiST, you need a Tcl procedure to serve as the operation. An input message contains the arguments and the output message, obviously, the result. The only 'problem' is one of picking a procedure which exposes what you want. If the code which needs to execute is not packaged up this way, then the 'problem' is to write it, similar to what I would need to do for my qw.tcl file which processes all form submissions. Obviously my job is much simpler because I only have one page to deal with. On the other hand, I have the same issues for the applications which use query-writer: forms/pages which select, organize and display information. One advantage of query-writer is it contains a lot of data model information, so it could be used to auto-generate TWiST configuration files and create simple select (one or many) Tcl API and also use the qw_* API for insert/update/delete operations. (Since query-writer gets type information directly from the database, typing can be automated.)
Working with a procedure defined via ad_proc actually presents very similar issues as an application page. ad_proc serves many purposes, but the way it works makes it harder to expose as a web service. This is why TWiST works the way it does: configuration is done via <ws>proc, which provides a wrapper for the internal API. Inside this wrapper, you can specify any amount of code. Maybe you could source a tcl page and do something with the resultant state. Whatever goes on inside the wrapper, you just need to return a list which represents the output message.
One point about typing with TWiST: more important is the structural typing, such as the names and order of the complexType elements, how many of each of the elements, can it be missing, is there a default, if missing, does this indicate it is 'nil', etc. Otherwise, your return document will just be a smashed together mess, but internally TWiST does not care about types, just structure. Essentially the type information is a description of the template: what is the document going to look like. Nobody would ever suggest we just run a tcl script and return a list of all the variables, the results need to be formatted, and this is what the type specification does: defines the non-data parts of the document. The wrapper code doesn't need to know anything about XML or any of this configuration, it just needs to return a list which will be formatted into a document.
When the TWiST client is done it will only handle document/literal, whatever is available now is in the wsclient package of tWSDL. I think everything is in a web page, and it requires the ssl module to be installed, at least to use ns_https.
