This should probably be added to the help for REGION. It seems to support it just fine!
TIP Specifies the text that displays as “balloon help” when the mouse cursor pauses over the control (PROP:ToolTip).
PROGRAM
MAP
END
Window WINDOW('Caption'),AT(,,395,224),GRAY,FONT('Microsoft Sans Serif',8)
BUTTON('&OK'),AT(291,201,41,14),USE(?OkButton),DEFAULT
BUTTON('&Cancel'),AT(333,201,42,14),USE(?CancelButton)
REGION,AT(23,24,141,148),USE(?MyRegion)
END
CODE
OPEN(Window)
?MyRegion{PROP:Tip} = 'Well what do you know...'
ACCEPT
END
Thanks @MarkGoldberg for the code snippet!