Well, I finally figured out the relationship by pure brute force: bounced objects off the side of the screen and used message boxes to output the values. Thank God I used pi/4 rads as the fovy (field of view y used in the D3DXMatrixPerspectiveFov), or it would have been nigh impossible to figure out. (for me, anyway)
If anybody else needs to calculate their screen bounds in world space, it goes like this:
Ymax = z * tan (fovy/2)
Xmax = Ymax * aspect ratio
This assumes that the camera is straight back along the z-axis. But for a 2D game using 3D meshes, the camera doesn't move, so simple trig is good enough.