There are a couple of ways to add the masking you want to ArtRage 2.5 Full edition.
The easiest way would be to create a new layer, paint using any of the tools the area you want to mask. Then from the layer menu you can select 'Make a stencil from this layer'. A new mask is created, with holes wherever there was paint on your layer. Delete the layer you used to paint on.
If you want, you can invert the stencil by right-clicking on it and selecting 'Invert Stencil'. Then the mask will have solid bits where you were painting, and holes where there were holes in the layer.
If you want it to act sorta like a Photoshop quickmask, make your layer about 30% opaque, and paint with red paint!
You can also work backward - use the paint-roller to fill your layer with paint, then use the eraser to make the holes for your mask. Create a stencil from layer, then invert it to match your layer. Delete (or hide) your masking layer.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"