I don't think there is a way to do this with out-of-the-box tools.
Wire duct is parametrically drawn; it doesn't exist as an editable block anywhere. The data used by ACADE to draw wire duct is held in an Excel file. You pick which wire duct you want, ACADE looks at the Excel sheet to see the data for that duct, which are really the instructions for drawing it, and then the program draws it for you. Wire duct comes in as a block, but it is actually drawn on demand when you need it. ACADE draws the lines and blocks them together as part of the overall command.
Terminals are actual block files that you can edit. The reason terminal blocks obscure what's behind them is because their drawing files have WIPEOUT entities inside them. It's the wipeout that does what you're wanting here, and there's no way to include one in a parametrically drawn object in ACADE.
That said, you can manually place a wipeout to obscure something that your wire duct sits on top of, for instance a mounting hole in the back panel. Enter WIPEOUT at the command line, pay attention to the prompts. Then, use DRAWORDER to set the wire duct to be in the front.
If you haven't used a WIPEOUT before, here's some things to keep in mind:
*WIPEOUTs can't have arcs or circular line segments of any kind
*set the wipeout frame so that it doesn't plot...this option is in the prompts as you go through the command
*there is a performance penalty with using WIPEOUTs...basically, the more vertices in the drawing that belong to a WIPEOUT, the bigger the drag on overall performance. This is especially noticeable when you have lots of WIPEOUTs and you start panning, zooming, and using REGEN or REDRAW commands. Try to use the simplest shape you can to hide something. A square or rectangle is the simplest, use these as much as possible. Example - obscuring a circle with a WIPEOUT: using a square as a WIPEOUT doesn't hug the circle closely, but it's easier for the computer to display. However, if you try to hug the circle closely by drawing a 24-sided polygon to use as a wipeout, it will exact a larger performance penalty. This won't be noticeable at first, but if you have a bunch of those hanging around, you're likely to see performance issues. Keep that in mind, before you go too crazy throwing WIPEOUTs around to obscure things that are in the background.
Hope this helps,
Jim Seefeldt
Electrical Engineering Technician