Alice, I should say that C. japonica usually produces aromatic, candy-like fruit but that the more commonly grown hybrid C. superba is sometimes bland until fully cooked. It depends on how strongly the C. speciosa genes are expressed in the hybrid. Either way, the flavor of all quinces intensifies and deepens upon sufficient cooking. I haven't used a pressure cooker for this purpose, so you'd have to see how well it works for yourself. It will achieve the high temperatures and keep the volatiles largely contained but the presence of oxygen may be necessary to develop the deep aroma that you liked so much with the jelly. I know that both heat and oxygen are required to convert the tannins into anthocyanins, so the same may be true for the development of some of the flavour/aroma compounds. Although you might be able to achieve that by first boiling the quinces in a pressure cooker and then aging the extract, so that it oxidizes with time. You could also perhaps convert your pressure cooker into a still, so that it let's in sufficient oxygen and also condenses the escaping volatiles. You can find instructions and pictures online showing how to do that.
If you manage to find some quinces with a good aroma when raw, you may also want to tincture some of them. In the Quince liqueur recipe below, Pseudocydonia sinensis (Karin fruit in Japan) was used but the Japanese have also historically used C. japonica in liqueurs, with it being the only Quince indigenous to Japan. Naturally, you'd want to omit the sugar in the recipe if it's intended for fragrance use.
Edited by Pears - 8/21/13 at 1:03am