I think the biggest issue is that the relationship between pitch attitude (angle) and rate of ascent/descent in aircraft is tenuous at best. During landing, for example, it is typical to have a healthy descent rate with the nose slightly up from level. I don't think I've ever seen any procedure with a fixed pitch angle, it pretty much always depends on the gross weight anyway.
Emergency descent maneuvers in the C-172 and I believe many other small aircraft actually involve a 45 degree roll angle, and whatever pitch angle achieves a desired speed (e.g. 120 kias in the C-172). This produces a tight downward spiral, the spiral helps because it moves lift off the vertical axis which lets you descend more quickly at a fixed speed - otherwise you'll speed up until the wings fall off. I suspect the 45 degrees above may have come from confusion over which axis it was around. I'm not sure what the exact justification is for the choice of 45 degrees but 60 degrees roll angle is usually considered sort of the upper limit (beyond that is technically aerobatics) so 45 is pretty commonly considered the "steep" extreme.
It looks like the procedure is different for the big boys. A quick google shows that the emergency descent procedure for the 747 is thrust closed, speedbrakes at flight position, gear down, make 320 kias. Pitch angle that produces would depend on weight but it's not going to be 45. No spiraling involved, but then 747 seems to (unsurprisingly) have a lower max bank angle (really max load factor) than smaller planes.
The point is you wouldn't even really pay attention to the pitch attitude, you'd pay attention to airspeed. The answer to "what should the pitch be" is pretty much always "whatever results in the airspeed you want."