I live next to a MUP that runs primarily through the undeveloped desert. It is nice to use this MUP becasue it leads to roads farther away from the city. There is little traffic on this path, (unlike the urban MUPs in town) and once I am off the MUP the roads open up and don't have traffic lights.

The problem is that the MUP has lots of rocks on it especially on the corners due to ATV/dirt bike users, despite signs posted restricting motorized vehicles. Since the trail is on BLM land the city and county can't (won't?) enforce the rules or clean the path.

Instead of (only) complaining I am trying to be part of the solution. I would like to buy/build a tow-behind sweeper that I could bring on the path to sweep away the rocks. I have found a few sweepers that are designed to tow behind a lawn mower, but these are for picking up leaves, not knocking rocks off a path. I think a simple design of dragging a section of chain link fence, similar to a groundskeeper's tool for a baseball infield might work, but I don't want to simply drag the rocks farther I want to knock them off the path entirely. Perhaps if I put an angled leading edge on the chain linked fence the rocks would deflect off? Will draging an apparatus like this damage the asphalt surface of the trail?

I am looking for suggestions or better ideas here.

I think you are on the right track. I grew up on a farm and the rake that we used to put the hay in a windrow was angled, as you say, to put the hay to the side of the rake...SO your chain fence idea or really stiff brisles of some kind,,,but I am thinking of a rotating mechanism that would actually sweep the rocks away. If you are talking big rocks, as in golf ball size, then you could use an angled blade and shove them off to the side.
I have sometimes thought the same thing you are thinking about some of the paths close by but never took the time to actually do anything.
Keep up the good work. .... ... ... ... .... ... ...peace

How about hiring a curling team wearing rollerblades and have them sweep in front of you? You can convince them it's good training.

Get a BOB trailer and a couple of wide brooms. Rig both brooms at an angle to the trailer and pull it behind your bike. When you're out sweeping the trails on your bike bug your eyes out and talk to yourself. Soon, everybody will know "Crazy Sweeper Guy."

Take a broom, the outdoor type, on your bike. Stop and sweep the areas as necessary. This will be the easiest to set up and give excellent results.

Write a letter to the authorities explaining that you are concerned about the safety violations on the MUP whereby cyclists/pedestrians can slip and fall on the loose surface resulting in huge liability claims.

Then again, better not do the sweeping yourself because you will become responsible for the condition of the surface!

Yea, and if you start sweeping...then others will come to expect it.
An angled rotating drum with broom heads on it ... ... ... peace