[horde] DIMP & mail_hdr preference

[Adam Tauno Williams]

Does DIMP not support the mail_hdr preference in preview?

I'm setting this via imp/config/prefs.local.php to a value and setting  
it to locked.  The headers appear in traditional view but this does  
not make the header appear in the DIMP preview [it does appear if I  
'open' the message].

Is this not supported or is there a way to toggle-on showing extra  
headers in preview?