Bernard,
I now have 4 gal of hibiscus-ginger wine in secondaries. I'm pretty new at this, so take these comments with a grain of salt, but I'll say that the hibiscus-ginger is looking like it will be one of my better wines.
Keller says to use H. sabdariffa. I don't have H. sabdariffa but I have three pretty big H. rosa-sinensis in pots, so that is what I used. Keller calls for 1-2oz of dried flowers/gal, and I went with 2oz/gal. I added 1.5tsp of ginger paste/gallon, and otherwise I pretty much followed the Keller recipe. I pick my flowers and freeze them without completely drying them, because it's just easier for me to count flowers than it is to completely dry them and weigh them out. However, I did that to a small quantity so I would know how many dried flowers/ounce. For me, the dried flowers number 80 flowers/ounce, so I'm using 160 flowers/gallon. I cut out the pistil/stamen structure before using them.
What I've found is that this wine ferments fairly slowly; when removing the straining bag, as you squeeze the bag you first get watery must, but as you squeeze it harder you get this stringy clear stuff that has the character and quality of glycerin. I have to think that helps with the body. The wine has a wonderful floral smell that is entirely absent from the fresh flowers. And it already tastes good, so it looks to be drinkable very young. I'm trying to be patient, though. I can post pics if you want.