The ability to simulation RMS envelope back propagation was added to the J-PARC XAL online model. This feature provides support for algorithms estimating upstream conditions from downstream data. Because of space charge and RF gap effects, implementing back propagation requires considerably more effort than simple matrix inversion. Thus, the new feature was implemented as a separate mechanism within the XAL online model. This strategy was taken to avoid side-effects, increase code readability, and to minimize any obfuscation to the software developer. Even so, significant refactoring to the original online model architecture was required to maintain compatibility with the existing simulation features. Quite noticeable is the new algorithm class hierarchy. In addition, the technique for modeling space charge effects needed to be refactored. We outline some of these details and show simulations using the new feature.