Coalporters

The Bethgon Coalporters are also used by private owners as well as railroads. I haven't seen them used for other purposes than coal. I guess it's due to the fact they can be only unloaded by a rotary dumper due to lack of chutes.

Technically you should use open hoppers--such as those made by Pennzee/Full Throttle or Micro-Trains--or gondolas like Micro-Trains or AZL make.   Open hoppers are preferred in most cases due to the gravity unloading capability.   As was mentioned, having to rotary dump a bethgon full of rocks wouldn't be very economical.

While some aluminum coal gons have been repurposed in *very recent* years and equipped with tarps to haul cattle feed, this is a fairly new practice.

Since you asked what *I* think about it, I would not load bethgons at a rock/gravel plant.   It would not be very realistic in my book.   I would use the 100T Full Throttle hoppers.   Very nice cars.

Only as a discussion point...

If I were you, I would lean toward the BNSF side of things for these cars.   The reason?   The most recent release of the UP brown cars from Full Throttle are of an older scheme, not anything you would typically see in the era where UP and BNSF coexist.    There *are* UP cars that were done with white lettering that ARE more appropriate, but they are hard to find.   HOWEVER, there are new and available releases from Full Throttle for two different BNSF cars and Santa Fe all within the past two years or so.  See:
www.zscalemonster.com/full_throttle/8000/ft-8008.jpg BNSF #1
www.zscalemonster.com/full_throttle/8000/ft-8007.jpg BNSF #2
www.zscalemonster.com/full_throttle/8000/ft-8013.jpg Santa Fe

Note, the photos above tend to show the cars carrying coal and that's certainly possible.   They do sometimes still carry coal (or more likely *coke*) these days, but a lot of them carry gravel or ballast as well.   Once the downturn in the coal demand happened in the late 2000's, the steel cars hauling coal moved on to other service so that the more efficient aluminum cars could give the railroads a better profit on the long haul coal routes.