The preferred way to load pull-ups is with a dip belt hung around the hips. This allows for variable loading from very little weight to multiple 45 pound plates, but more relevant to this question, it centers the weight on your hips instead of the shoulders. This means it doesn't change the angle you're pulling at, it simply makes you heavier. It should maintain the stress on your abs that you're looking for.

Ross Enamait has video instructions (and static images) on how to put a dip belt together from hardware store materials. It's quite a straightforward, cheap, and useful tool. You can also buy a specialized dip belt, which is still useful but not cheap.

The preferred way to load pull-ups is with a dip belt hung around the hips. This allows for variable loading from very little weight to multiple 45 pound plates, but more relevant to this question, it centers the weight on your hips instead of the shoulders. This means it doesn't change the angle you're pulling at, it simply makes you heavier. It should maintain the stress on your abs that you're looking for.

Ross Enamait has video instructions (and static images) on how to put a dip belt together from hardware store materials. It's quite a straightforward, cheap, useful tool. You can also buy a specialized dip belt, which is still useful but not cheap.