White Elmers glue.....don't laugh.
completely cover the back of the patch place it and clamp it down, weight it down HOWEVER be careful of glue squeeze out it will not wash out of the canvas.
Second process.......mix Elmers glue about 60% glue and 40% water. Coat the back of the patch and a masked off area on the jacket slightly smaller than the patch.
Allow the coating to completely dry on both surfaces, re-coat if necessary until you have a semi shiny coating .
Elmers is 'white' glue is PVA, and remains water soluble. The 'Yellow' glue as well as PVA contains aliphatic resins which cross link as the water evaporates rendering it 'water resistant' – and when heated even with a household iron the Elmers glue will bond to itself.
The heat required for this is relatively low.
I have used this process to attach a balsa wood skin to expanded bead foam wings (RC aircraft) with just an iron.
The best suggestion I can give you is TEST TEST TEST.....buy some duck cloth and some leather and work on the technique and heat.
Lastly I have in the past applied patches via the applique' process on an embroidery machine.
I know they do not want the patches sewn however if you stick the patch to the jacket with fabric tac. and properly digitize the sew stitch you can lay it down just in the border stitching of the patch (match the thread color) and you can avoid the typical over the edge stitching and pretty much make the attachment stitching invisible .
moosegoldberg