Chris,
Sorry, perhaps I'm just being thick, but how does a 90 degree insert allow you to do chamfers without it first being rotated to 45 degrees?
I use triangular TCMT inserts and one of my turning tools has the insert set at a 15 degreee angle, so that the cutting edge is at 45 degreees to the axis of the lathe. In this arrangement, you use the edge of the tool (or part of it) as a "forming" cutter (even thought it is a straight edge) and when used in conjunction with a QCTP makes doing chamfers dead easy. Obviously, I have both left and right hand versions for chamfering both ways. You could use one of these tools mounted along the axis of the lathe for your internal chamfering if the job prevented "normal" mounting (at right angles to the lathe axis).
Regards Tom