I don't think there are any more references with regards to accessing and controlling the PI Trend Control in Excel via VBA.
When you say selecting a portion of the trend, are you thinking of zooming into the portion? or you are thinking of using trend cursors to identify the portion?
I don't know if you are familiar with the old PIArchiveEditor but it was a handy tool. However, it only writes to one of the nodes on a PI Collective. The trend portion of that application (PITrend.ocx) would allow you to 'draw' a box on the trend and return any points within the box. I thought about trying to use that active-X object, but I have no documentation on that and updating to the newer PITrendXL seemed preferrable.
The zoom event will reduce the viewable area. If I could get all of the archived points in that area, I could use them to highlight cells in a spreadsheet for editing.