I'd be surprised if you can achieve this, tbh. While I understand what you want to do, what you're asking the camera to render doesn't make any sense (to me, anyway). I mean, in the real world, if you're sat on a chair looking forward and you want to change the plane of your vision to be higher, you have to stand up. It seems to me that what you're wanting to do is render the view seen while standing AND sitting down.
Maybe you don't understand what I'm trying to say - I'm not always good at explaining things. :/ Maybe you can do some trickery by fiddling with the perspective matrix but I'm betting the result won't be what you're after.
Maybe you don't understand what I'm trying to say - I'm not always good at explaining things. :/ Maybe you can do some trickery by fiddling with the perspective matrix but I'm betting the result won't be what you're after.