No matter how hard I try to think outside the square (or should that be "think on the taper") the best workaround I can think of utilizes sidelites hidden by columns - regular ones on the exterior & profiled columns inside. It'll necessitate some cleanup work in 2D but should look spot-on in 3D.
First I inserted the correct sized window - if it already has sidelites I'm up the creek
Once it's inserted, add sidelites equal in width to the taper in the reveals.
So I didn't have to rely on subtracting the window Frame Offset c + Frame Depth b from the overall wall width I adjusted c then inserted a 1:20 SmartView of the window & adjoining walls on a worksheet reserved for "scratching around but leaving a paper trail". I then dimensioned the relevant parts & set their value Scale to 0.2
In Element Manager, Columns add a new parametric column using Type = Custom
Select Profile, then add a new profile starting at a point so that you can use the Close option to create the non-ortho segment.
Make sure you create the profile in a clockwise direction. My first attempt was anticlockwise but when I pressed the Close button to create the non-ortho segment Env suggested "Note: Closing will cause a self-intersecting polygon" and refused to add the final segment.
Make certain you uncheck Uniform Scale & enter the overall dimensions dx1 & dy1
Insert the "column", adjust it's height, elevate as required then nudge it into position. Mirror it about the window centreline.
Add a suitably sized rectangular column externally & repeat
In 3D View use Materials Paintbrush to "paint" the external reveals. If it's a stone texture it's a mongrel trying to map them correctly!
Much easier if the internal tapered reveals are rendered finish.
After inserting hidden line or patterned SmartViews on worksheets there'll be quite a bit of cleanup work involved but . . .
btw, let us know how it goes, especially if the openings have arched tops - that could be challenging!