The center of gravity is your metric. If the center falls within the span of the rears you should be good to go. Anything below the CG and within the confines or the rears helps with stability. The lower the mass the less required to get the desired result. Loading tires is about as low as you can get.
Couple of other things to consider:
1. Hitting a bump transfers the CG instantaneously and can result in an unpredictable result. Speed hinders since the faster you go and the larger the bump, the more energy involved to overcome your normal CG.
2. Sliding down the hill is always a problem, especially on wet grass like is encountered when you are mowing in the mornings before it gets too hot. A loaded R1 on a suitable soil (soft enough for lugs to penetrate and hold), reasonable hill side (30 degree is getting pretty risky with no other gotchas) may hold just fine. Otherwise you are asking for trouble.
