I don't know of a way either. What I have done in the past when I was looking to remove all of the CUSTOM1 entries was to load all of the favorite lists into ARC536PRO (I've also done it with ARC Patrol), then print all systems to a pdf file, and then search the pdf file via CTRL+F for "CUSTOM1". Once I find them in the pdf document then I go back to Sentinel and correct/change them.
Actually, you can do that via the Favorites Editor in ProScan. Either by a find & replace signally, or a directed find & replace all.
As far as a "bulk update" goes, if you are looking to make a change or correction in a group of frequencies, or talkgroups, in a given department, you can update one of the lines, then copy the selected field that was changed to the windows clipboard. Then select the range of channels/tgids that need changing. (If all are in a contiguous list, hold down the <Shift> key while clicking on the first and then the last item to select them as well as all in between. If not contiguous, then hold down the <CTRL> key while seelcting the ones to change. After all are selected (they would be highlighted now in your list), paste the setting you copied before into all of them at once.