Plumbing Question: Shower Escutcheon Removal

A friend asked me to take a look at their shower valve. He said it's sticking and hard to operate. He was right.
Thinking I'd just replace a cartridge for him, discovered that I couldn't unscrew the escutcheon. Don't know whether or not you can see it in the photo but looks like previous homeowner had tried to remove it. I see a couple of cracks around the edge where they used vice grips or some such. I didn't spend more than a couple minutes on it but tried both a strap wrench and channel locks to no avail. Didn't have time to get into a valve replacement if I twisted something loose. Told him I do have a couple of hours tomorrow and would have another go. Dishwasher's on the back side so I can cut a hole for replacement, if things get ugly.
You plumbers have any tricks to removing these things when they're frozen on?
Thanks.
http://sdudley.com/images/shower-valve.jpg
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Whats the "brand" of the device ? Is there a name or logo on the handle or shower-head ? I'd look up their website and see if there were any hints on how to get the thing off..(screw off ..pry off or WHY)

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No print or identification of any kind. That's what I was hoping for too.
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