Don't worry- it's quite normal to stash contraption state in auxiliary hidden widgets. The contents of such widgets is distinct between contraption instances, but the background image of contraptions is shared among all instances; pasting directly onto the contraption background will cause issues if you have more than one sprite. The conventional solution would be to add a canvas widget to the contraption and draw on that instead. You'll also need to configure non-zero margins for the contraption to ensure that the canvas automatically stretches and resizes properly along with it. Both the contraption instance and the canvas will need to be set to "show transparent" if you want objects behind the sprite to show through.
Since you intend to redraw the contents of the canvas frequently, you may be able to mark it as volatile, which will ensure that having many sprite instances won't bloat deck size with unnecessary copies of sprite frames. If you aren't already doing something similar, you may find it useful to mark the canvas as "animated" so that it will automatically bubble view[] events to the contraption instance at 60fps and "locked" so that it doesn't inherit the default click-and-drag-to-scribble behavior of canvases. (Be aware that "me" will be bound to the original target of the view[] event, not necessarily the contraption itself!)
The Sokoban example deck contains a contraption called "mover" which works similarly in some ways to your "sprite" design, and the Path example deck has another variant called "follower". You might find these useful to reference.
Does any of this point you in the right direction?