T.S. O'Rama never fails to post fascinating and thought provoking things. My mind bubbles with all sorts of thoughts all the time and occasionally one struggles to the high-surface tension top of the liquid and explodes with amazing display, usually over some triviality. Not to break that sequence, I must comment on this comment Mr. O'Rama offers.
Perhaps the answer is this: everything but humility. If the Medjugorje messages said, "humble yourselves before your family & neighbor" instead of the unceasing requests to pray, perhaps that would be off-limits as a demonic strategy.
I think I would say, put no good thing beyond Satan's power. That is, if praying the Rosary will keep you at the same level of prayer and cause you not to advance, that is a victory for him. He would encourage you to be very devout in your prayer of the Rosary. If humility seems good, he can make it a marketable commodity, and suddenly people who were full of humility are measuring themselves against others and against a false standard. Satan can use all morally good and neutral things to ill effect. We can be tempted to spend hours round-the-clock before the Blessed Sacrament, indeed a good thing, to keep us from supporting our families and doing our duties in our married vocations. So Jesus told us not to judge by appearances or by what was said ("wolves in sheep’s clothing.") but "by their fruits you shall know them."
Now this becomes an extremely tricky business. Take the matter of the forthcoming canonization of Josemaria Escriva. I have read elsewhere that he encouraged practices that would certainly seem to overstep the bounds of what modern sensibilities could entertain or accept. But do a majority of cooperators engage in these? (Did he indeed encourage any such thing or are these scurrilous rumors? I do not have enough facts at my disposal to say for certain.) What are the fruits?
That is why I simply await the full investigation of anything--apparitions, sainthood, acceptable practices and prayers. Presumably both greater numbers of people and people with a great deal more experience examine these things before they are approved. I think we fall into a trap making assumptions about what Satan can and cannot do and we do better to err on the side of accepting what is traditionally taught. These new apparitions may not make their meaning known for some time. It took a long while before we knew and understood the full revelation of Fatima. Lourdes was not well accepted immediately in its time, and we may not yet have truly absorbed all that is there for us.
Thus my caution. Satan is a lot smarter than we are, with thousands of years of tempting and experience with human souls at his fingertips, I would venture to guess that there is almost nothing that he cannot corrupt, at least in practice. Obviously he cannot make invalid a properly consecrated Eucharist, but he can lead us to believe the lies many modernists would tell of it.
The best thing to do--set your eyes on Christ and do all that you do not for hope of heaven or fear of hell, but from pure love of God. You might be led astray, but it seems unlikely that He would allow it.