thank you very much ajshort!
your suggestion using '$controller->getViewer('index')->process($controller)' works like a charm.
maybe once you have time to leave some comment on what this line exactly does, but for now i'm very happy with it.
for a short explanation on why i don't want to link directly to my projectpages:
i'm using a kind of filter to categorize my projectpages, having each projectpage linked to some filterpages.
each filterpage renders a thumbnail of the projectpages linked to it, which leads to the projectpage.
problem is, the projectpages itself are located elsewhere in the sitetree, so if would link the thumbnail there directly, the navigation wouldn't reflect the path of the filterpage i came frome any more.
that's why i decided to render the projectpage using a method of my filterpage (using an url like 'filterpage/project/id') which allows me to stay on my filterpage and have it highlighted properly in the navigation.
i hope that's somewhat comprehensible!