3D over 2D

There's been a lot of talk recently about using 2D commands in Blitz3D (or more specifically, using 3D objects to replicate a 2D HUD). But what I'd like to do is mostly 2D with a few 3D effects overlayed on top (like Baldur's Gate 2 did).

So far I've come to the conclusion that you can't do that in Blitz3D as calling Renderworld always clears the buffer. But I'd be happy if someone could tell me otherwise.

CameraClsMode camera,cls_color,cls_zbuffer

Parameters

camera - camera handle
cls_color - true to clear the color buffer, false not to
cls_zbuffer - true to clear the z-buffer, false not to

Thank you very much :-)