Drew,

You could drive it at a slower speed, but that clock would have to derived,using a divider, from the MCLKx. Otherwise you can not guarantee that it will be synchronous.(If you try to drive this pin with diiferent crystals there is no guarantee that they will be synchronous to each other because of phase differences, tolerances, etc.)
You could try to use the same crystal to drive the uprocessor, assuming that you used the timer of the uprocessor,but then you would then have to deal with the slowness of execution speed of the uprocessor, because it is driven from such a slow clock.

Jacob