What would the radiation pattern look like if I used a 1/4th wave 2 meter mag-mount antenna on 70cm with an HT.
antenna-theory.com provides some nice images of the ideal case. A half-wave dipole in free space along the vertical has a radiation pattern like this:
A 1/2 wave antenna on 2m is a 3/2 wave antenna on 70cm. That radiation pattern looks like this:
Of course you are not using a dipole, but a vertical. A vertical on an ideal ground plane has an identical pattern to a dipole, but with the lower half removed. On a ground plane of limited size and conductivity, some of the low angle radiation is lost, but otherwise the pattern is pretty much the same.
In a mobile installation, the ground plane can be very far from ideal, and the radiation pattern will depend more on the ground "plane" than the antenna, so the actual pattern will not resemble these images at all.