Howdy
I use Interchange for my switchlist generation. I asked about randomizing mine output on their mailing list, and got the response below. Some form of this may work for you guys using RailOp as well.

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First, establish what the minimum number of shipments for each
individual mine will be and then create a single shipment with that
number of cars and a frequency of “daily”. Then establish individual
shipments from that mine for each additional shipment that could be
possible.

For example, lets assume you want to have 10 shipments +/-30% each
day. This means the number of shipments could range from 7 to 13.

Create a single shipment requiring 7 cars and a frequency of daily.

Then create 6 additional shipments each requiring 1 car with a
“random” frequency. When you select random, a slider will be
displayed where you can adjust the likelyhood of that shipment being
executed. This random is not daily, it is determined each time a
train that could execute the shipment is run. If you only run a
single daily train that could possibly haul shipments from the mine,
then the randomness would in fact be daily. If you have more than one
train that could haul the shipment then there is the risk that the
number of shipments could exceed the 13. Reducing the random slider
towards zero will minimize this problem. Give each of the “random”
shipments a different percentage value or you risk all of them
executing at the same time.

The way the random function works is that a random number is picked.
Any shipment that has a frequency value set that that is equal to or
greater than the random number will be executed. So if you select 10,
20, 30, 40, 50, and 60 for your values and the random number is 25,
the shipments with the values of 30, 40, 50, and 60 will all execute.