Dear Tony K and Mat
thanks a lot for your thoughtful replies. After tinkering some (ahem: a lot), I know that I could realise a filter along the lines of what Mat suggested, one for each project. It is not precisely what I was after, so I would like to make this my plan B, the project plugin by Nicolas has many more options, a clean design and is a lot more open to changes in the structure of projects. Obviously a lot of thought has gone into its creation, and that level of refinement would be very hard or impossible for me to achieve.
I found Nicolas' thread in which he announced the release of his plugin and I will try to highlight that it would be useful to allow subordinate notes under the project's name that are not automatically treated as tasks, which is what I was trying to say in my first post and obviously failed.
Thanks nonetheless, Mat, for teaching me something about filters!