A small amount of ingested aspen will very likely not bother her at all. Remember their digestive acids easily deal with flesh, bone, fur and teeth. However, if you are concerned, simply lay a pad of newspaper down over the middle portion of her enclosure and feed on that area. Once she's eaten and retreated back into her hide to digest you can slip the newpaper out.

Ball pythons can tend to be pretty prey specific and very shy. She may feel far more secure hunting from her hide and in her own home and taking the prey she at this point identifies as her food source. Personally, I'd do as Derek said and stick with mice for now (you can feed multiples to achieve the goal of enough total volume) and switch her to rats later if you want. Many ball pythons though do just fine their entire life on multiple mice per feeding.