I'm sorry to say that I cannot duplicate what you are describing in v14 or v15.
I created a panel with Post_Display logic that does PRINT 'CAPTION'("new title"),
I process the panel using PROCESS "mypanel","mylib" and the title in the title bar changes.
I also created a panel using a Nomads object that does PRINT 'CAPTION'("new title") in the POSTLOAD() logic.
I instantiate the object and process the panel with myobj'Process(), and the title bar changes.
The way it works is this:
IF there is a title bar on the panel, the SignalCaptionChg option is set to -1109. This means that whenever a PRINT 'CAPTION'() is executed a CTL value of -1109 is generated.
When the -1109 CTL is detected, the new caption is retrieved using a MULTI_LINE READ 0, and the result is redrawn as a font image.
All of this should happen automatically if you have created a fonted text control called PX_TitleBar_Caption on your title bar.