I have filled out some drilled holes in wedges with "tool dip". It worked really well and allowed me the get the weight exactly where I wanted. I've done it with and without tungsten powder for different weighting. I'd think you could do similar with your Pings. I think the tool dip stuff would be better and easier to work with than Flexseal.
I've also thought about 2 part expandable foam as an option if I could do small enough batches. Not sure how well it would mix with tungsten powder though.
