Alternatives to Render Textures

Hey guys I'd appreciate some help on a problem I'm having. We need to render multiple objects inside of "windows" that can be dragged around the screen while staying in a 3D perspective. At first we thought we could easily achieve this using Render-to-Texture. Just have a textured panel inside of the "window" that renders from a second camera pointed at our desired object. However, we believe this would drastically hurt our performance since we are looking at having a large number of objects (10+) rendering on every update.

We looked at a number of alternatives including the use of the stencil buffer. This would allow us to keep the object inside of the window, but still cull any part of the object that extended past the boundaries. However, keeping the object in 3D space and moving the object along with the window means that the perspective on the object is changed. So, we essentially require a solution that can render the object separately inside of the window so that the placement changes when dragged but not the view (exactly what Render-to-Texture is perfect for).

Has anyone had experience with a similar problem in Unity or have any ideas? We are essentially looking for either a way to increase the performance of the Render-to-Texture method or a similar way to cut and paste from viewports.

Thank You