Can I hide the rewind button in the iOS Lock Screen Music controls?

OUTDATED:

I think the short answer is no. The documentation says this -

"You do not have direct control over which information is displayed, or its formatting. You set the values of the now playing info center dictionary according to the information you want to provide to the system. The system, or the connected accessory, handles the information’s display in a consistent manner for all apps."

As of iOS 7.1 there is a way to customize the controls available in the lock screen and command center (and probably many other accessories): MPRemoteCommandCenter.

Regarding your question you can do the following:

[MPRemoteCommandCenter sharedCommandCenter].previousTrackCommand.enabled = NO;
[MPRemoteCommandCenter sharedCommandCenter].skipBackwardCommand.enabled = NO;
[MPRemoteCommandCenter sharedCommandCenter].seekBackwardCommand.enabled = NO;

// You must also register for any other command in order to take control
// of the command center, or else disabling other commands does not work.
// For example:
[MPRemoteCommandCenter sharedCommandCenter].playCommand.enabled = YES;
[[MPRemoteCommandCenter sharedCommandCenter].playCommand addTarget:self action:@selector(play)];

Please see this Stack Overflow answer for more general information about MPRemoteCommandCenter.