Why are you concerned with this particular devotion being scriptural? I personally believe it is (the 40 days in the desert), but does it really matter? As Christians, we all carry the cross of Christ, as human beings we all carry our own personal crosses.
Our puny little sufferings (i.e. giving up the drink, or salt, or caffeine, or whatever) is nothing compared to what Our Lord endured. It's a good reminder and really does bring folks closer to Jesus, and not just Catholic folks.
I see the kids with those WWJD bracelets. They're not in the Bible, but I'm sure some teenagers are reminded of Christ's love and perfection by those bracelets and a result do not engage in the sin which is all around them.
Likewise, when we are reminded of Christ's love for us through our own personal sacrifices, we receive additional grace which brings all of us sinners closer to Christ.
If a Baptist Pastor said that all kids were obligated to wear the bracelets, I wouls have a problem.