TSS becomes semi-irrelevant in such circumstances. It’s specifically the neuromuscular demand of the session that’s increasing, and TSS is more global than that (factoring in cardiorespiratory and metabolic demands). Heart rate will certainly do a better job than pace or power of reflecting this increased demand, but changing the workout to achieve the predicted TSS is not the best way to adjust to the circumstances. Even though you’re going slower, the session is already harder (i.e., more neuromuscularly taxing), and by lengthening it you’re making it harder still. In truth, it’s just plain impossible to make any adjustment that will make a run on snow and ice “the same” as a run in perfect conditions. It’s one of those it-is-what-it-is kinds of things. If it were me, I would go mainly by RPE, slowing down as much as necessary to make the run on snow and ice feel no harder than a run in perfect conditions, and complete the prescribed duration.