Create a square transparent canvas
Add your (circular) logo
Apply Filter>Distort>Polar>Polar To Rectangular
Image>Image Rotation>Flip Canvas Vertical
Image>Image Rotation>Flip Canvas Horizontal
Copy (CTRL-C)
Paste (CTRL-V) into your equirectangular panorama
Resize to fit the full width
Resize the height as desired
Place at the bottom of your image