I can't argue the logic of having a fat 'cushion' - though it would be a tough one to prove either way.
But there are practical issues: first, to be sure you've installed enough local fat to do the job, you'd need to put on several stone overall, quickly. Might not kill you, but it'll certainly damage you.
But the key issue is this: currently, your weight is stable, and you can keep it that way over the weeks of RT. Once you starting pigging out, your weight will vary significantly over the weeks, especially as your appetite (and effort) may flag over the course of treatment. If your weight varies, your prostate 'cushion' will vary. This and the varying pressure of fat elsewhere means your prostate will move within you. So pinpointing the beam (they usually use tattoos these days) will be unreliable. And you'll end up with exactly what you set out to avoid. Only more so. Quite possibly giving maximum dose to a slightly shifted rectum.
Eat healthy, be sure your bowels work regularly, and follow fluid intake instructions to the letter (and the second). That's the best we can all do.