As you alluded to, your gastric emptying is already going to slow to a near stop on a fast. If nothing comes in, nothing moves (and is instead just ever more extensively scavenged for ever-smaller amounts of useful nutrients).
The larger concern is that semaglutide works, as I understand it, by increasing the amount of insulin your body produces, but insulin production stops fat burning for as long as it’s in the bloodstream, so I can’t see how that would be helpful.