I've read everything I could find and I still have a problem...how do I determine which parameters of a stored procedure are output parameters in a .lpt template? I guess it's something I'm missing in SPCallParameterDefinition, but I can't see what.

Isn't it annoying, when, after staring at a problem for a considerable time, you finally ask a question on a forum and almost immediately spot the answer for yourself.

I have now found the 'RelatedParameter' and have sorted things out (and I'm feeling like an idiot)

Like we say in holland: "Every start is difficult". (For dutch: that's a very bad translation, I know )