Hi Jon,
AFAIK, and I do K quite a lot in this area, there is no way to pass stuff from the DocumentGenerator class into a template in the way you describe. The API is what it is, and the templates don't take parameters.
You can pull the project constants out of the database, but it's a little tricky.
Project constants are stored in t_rtf, which is one of those messy hacks that the EA schema is so fond of. It has two columns, Type and Template, and project constants are stored in a row where Type='ProjectOpts' and Template='name1=value1;name2=value2;...;'.
Extracting the specific constant you're after should be doable with a bit of SQL-fu.
So you can't prompt for a value and then pass it to the template through the API, but you can prompt for a value, store it in t_rtf, and then pull it from there into your SQL fragment.
HTH,
/Uffe