Is there anything wrong with a Catholic rock band playing harder-style rock music, but with Catholic teachings/doctrine for lyrics? I know such music shouldn’t be played in Mass, but what about as an outreach to the youth, who listen to rock music? This would help bring them from the sinful lyrics and expose them to the truth.
There is nothing inherently wrong with any form of music provided it is played in its proper setting. As you mention, such music would not be appropriate as liturgical music; but for general praise-and-worship events outside of church and for youth ministry, it would be fine to seek to sanctify that which interests teens and young adults by providing music they like that incorporates Christian themes.
As a side note, one does not have to use explicitly Catholic or explicitly Christian lyrics in order to provide wholesome rock music. So long as the lyrics are within the bounds of Catholic morality, they do not have to explicitly refer to Catholicism in particular or Christianity in general. For example, some purely secular songs have been huge hits but have not transgressed basic morality. Examples off the top of my head would be Bette Midler’s “The Rose,” Sarah McLachlan’s “I Will Remember You,” Celine Dion’s “My Heart Will Go On,” and Ben E. King’s “Stand By Me.”