Yes, absolutely. As a supposed leader of others, observing the fast and feast of a pagan religion, Islam, specifically for the purpose of being holy by fasting from "food, water, sex, etc., from dawn to dusk" is satanic.
Now, fasting for holiness is Biblical, but there was usually an intent specified in the fast (ie: the removal of demons, etc.) New Testament fasts are not laws, there is no requirement in Christianity for fasts, especially of "food, water or sex". Ramadan is a legal requirement binding on Muslims. I know of a Muslim woman who struggles every Ramadan because the fast dangerously effects her body. She cannot give what her body needs to survive or she'll be considered "unholy".
The connection to the Galatian-style heresy should be obvious. This is works-salvation plain and simple. If ever there was an example of the dramatic difference between Christianity and other religions this is it and far from engaging in this gnostic heresy where abstaining from food, water and sex (especially) makes one somehow more spiritual or holy, Christians should note that Christ and Paul told us otherwise.
"Our main purpose for participating will be our own spiritual growth, health, learning, and maturity..."
His purpose, expressly stated on his website, is for spiritual growth, health, learning and maturity. Well, if engaging in the practices of pagan religions provides spiritual growth, why did the apostle Paul get so upset at the Galatians for engaging in Judaism? After all what's a little cut of the flesh? What was wrong with the Corinthians seeking the embrace of temple prostitutes? Surely those things were intended likewise for "spiritual growth, health, learning and maturity" in their religions. Why doesn't McLaren engage in sacrifices to Kali too while he's at it?
Yes, Paul did tell us to not worry about feasts and where foods came from, but this is far different than engaging in a practice that is specifically contrary to the foundation of our faith, freedom from the biding of man-made laws in Christ.
Therefore, by willingly engaging in this pagan ritual wherein one binds himself to the laws of a false god and religion, McLaren is truly "satanic". Not only that, but as a pastor and teacher of others, he is by his own actions encouraging believers to follow his example and thus leading others into darkness. Satanic fits the bill perfectly.