Hi Dave,

Thanks for reaching out with your question. First some background on seed 'scarification'.

A hard seed coating is a form of dormancy that can help plants optimize their germination success rates.  The hard seed coat keeps water and oxygen from reaching the seed easily and hence keeping it from germinating.  It is thought this keeps all of the seeds from one season from sprouting at the same time thereby spreading out the chance that conditions will be right for the seedlings to flourish.

There are many ways to scarify ie: break the seed coat barrier.  Recommendations vary from using sandpaper to thin the coat to a sharp knife to nick it to actually cutting off a section using a knife or nail clippers.  Sometimes soaking the seeds is all that is needed or is advised as then making the nick is easier.

The goal of scarification is to break through the seed coat without damaging the germ tissue itself.  Much of a seed is composed of nutrients that provide the food for the initial root formation and development of the cotyledons - those first two leaves that a seed produces.  These leaves then begin to photosynthesis and add more fuel for the seedling to continue growing.  Cutting into this part of the seed is not a problem but nicking the germ part of the seed is.

So your question is a very good one - how to open the seed coat without harming the plant embryo inside.  I could not find out how thick the coat is on a hibiscus seed.  Typically the color of the seed differs from the color of the coat so you might try slowly trimming the outside to see if you can determine when you are through the coat.  Or try cutting one of the seeds in half so you can see the thickness.  For some seeds you can tell where the seed attached to the flower ovary - think of that area on the inner curve of a bean.  This is where the plant embryo is located so you can then cut the seed coat on the opposite side.  I'm not familiar with hibiscus seeds but perhaps you can examine them to see if you can discern such a spot.

While this link takes you to "Gardening Know How" and not an academic website, it had the best images I found - and also a very good article that is written by a Harvard PhD plant scientist.

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/nicking-plant-seeds.htm

I hope this helps and may your hibiscus seeds thrive!