How to make a jellyish sauce for plate design
Hmmm. this will be tough. Most sauces that are dolloped and smeared are based on heavily reduced vegetable base, or a bean or legume, or a 'mayonnaise' or aioli base. 

To get a thick, smearable sauce with jelly or gelatin is tough, as it will want to 'set' to thicken, which will not allow smearing.

There are new thickeners like Xantham Gum and Carageenan that might help. Cook's Illustrated has a nice piece on Xantham Gum and alternate uses here:

Xantham Gum at CI