I have found a magic method of doing tip cuttings. I have posted it on the Impatiens forum as that was the plant I tried it with first, but it works for a wide variety of plants, especially those that dry out easily. There is a photograph.
I've used similar products for cuttings, as well. Works very well on houseplants and other tender stock. I've never tried it on woody material.
Just did some cuttings of Pereskia aculeata var. Godseffiana' in soil moist. I had wondered about doing this before but not till after I read your post did I actually try it. I put 4 cuttings in it and 9 cuttings in soil to see which roots faster.
I wouldn't use Hydrogel with Cacti (Pereskia), Succulents, Orchids or other types of plant that prefer predominantly dry conditions. The danger is that they will have too much water and rot.
Nice post, Ian. Here is a link.
Here is a link that might be useful: IMPATIENS CUTTINGS USING HYDROGEL