As far as I can tell you can cleanly add resistance to the sandbox by making it one of the settings. Something like a behavior section where you can set her to behaviors ranging from "stiff" (actively holding herself in the position you place her in, no restraints needed once the position is set), to "resisting" (her actively fleeing as much as she is able and attempting to escape her restraints - though actually allowing escape would probably require a complex system). I imagine the middle points include "limp" (current behavior) and "squirming" (small movements that make tying more difficult and cause her to move around in bondage but don't really constitute attempts to escape). This being a behavior setting that can be changed during the game also supports "RP scenarios" where you tie her up freely and then watch her squirm.
As for reactions, "dynamic reactions" is definitely difficult but adding a setting for the player to have her facial expressions be one of neutral, happy, horny, sad, angry, and afraid would greatly expand the range of emotional tones that players can set.
Final thoughts: a handful of anchor points would be nice, i.e. being able to "attach" her hips or head to the fuck machine so that when it's running it actually fucks her, or being able to attach ropes to her hair without bands around her whole head.