The HTC Security (PIN/Pass.) Lock Screen timeout limit is 15 minutes max. No power button option.
The stock HTC Music player has a lock screen widget that can be used for simple forward, backward, play/pause functions. On the lock screen, prior to password or PIN entry.
Also, when using the stock HTC Music player, if you have Music set as one of your quick launch icons (the 4 application shortcuts on the bottom of your homescreen and lockscreen), you can bypass password and PIN entry and shortcut into Music.
Password/PIN is prompted for when you try to back out of Music into the phone.
Toggle for this can be found within: Settings -> Security -> Unlock Music automatically
Which carrier are you with? Did they include HTC's 'Car' Application in your software?
The 'Car' Application has convenient Music controls for the stock HTC player. Car used to have the option to bypass password and PIN entry, but it looks like that bypass option might be gone now. Shortcuts to other applications can be added within Car.
You might have to look at applications in the the Play store for other options, especially if you are using a Music player other than HTC's. Maybe someone out there has a suggestion or two.
Skip Lock or
Delayed Lock might work if you are using Bluetooth in the vehicle, however, beware the application permissions might be excessive.