Minimal working frame rate for H264 codec
I'm not sure how it will behave at very low frame rates, but it is worth pointing out this would also limit your options on how and when you could change frames since they would have to follow on the clock cycles.  What is more likely to work in this case is a long keyframe interval.  The majority of frames in a compression like H.264 only store the changes from the previous frame.  In the case of a still image, the compression ratios will be huge because very little (no) change occurs between frames.  I'm not sure that you would really pick up enough of a savings from dropping the frame rate to be worth the loss of control over when you can make a change to the frame.

The best bet would be to try it with your media and see the results.  Compression is a highly content dependent thing and the best quality and compression for any particular clip is going to depend a lot on the nature of that clip, so trial is still the best way to test it out.