I have a LOT of zucchini to use up and want to try Rose’s recipe “Many-Splendored Quick Bread” (RHC page 131) .  It calls for unflavored instant oatmeal. I never have instant oatmeal here. I either buy coarse oats or quick cooking (which is different from instant). Can I just put the quantity of dry quick cooking oatmeal in the blender to make it a finer texture or will the resulting more floury texture negatively affect the cake?

If it were me, I would give it a shot. As far as I can tell, the main differences besides cooking times in these oatmeals is the finished texture of the cooked oatmeal—instant being the smoothest (and sometimes gluey). Plus is sounds like you have plenty of zucchini to play around with. The Bread bible also has a zucchini bread recipe. I always doctor it up with a few more spices as I find zucchini somewhat bland.