Draw one texture on to another? Get Set Pixels.

Hi there. I am familiar with Get and Set Pixel functions but I wish to use them in an unorthodox way.

Lets say you have 2 textures. Texture A and Texture B.

Texture A: width 1000, height 1000 Color "brick pattern"

Texture B: width 50, height 50 Color "One black circle"

Now if texture A is applied to a and I shoot a ray at it and get UV cords from the plane and wish to draw the 50x50 circle there. Is there a standard way of handling this sort of of thing that is not too slow?

Thank you.