His Jabsco "Leak-O-Matic" is self priming and even though it probably wouldn't hurt the motor to run in reverse I still wouldn't try it for several reasons.
The nut on the end of the shaft could back off, ruining the pumphousing.
The pump efficiency would be greatly reduced in reverse, if it pumped at all. You would probably fill the tank quicker by flushing the head.
If the pump did work in reverse would you really want to risk putting the blackwater tank under pressure?
On our RV we have a water flush attachment that connects to the discharge fitting and flushes the tank with fresh water during tank discharge. It would not be all that difficult to plumb a similar flushing device into the deck pump out of a marine sanitation system.
Or, of you have an extra fitting on your tank (or weld one on with a cheapo Harbor Freight plastic welder) you could attach an adapter to add water to the tank either from an auxiliary wash down pump or muni water.
Still seems like a complicated fix for a noncritical problem to me though.
Just to add to these excellent points. These macerators are designed with a directional flow head and means they are designed to flow best in just one direction. Sucking water in through the outlet and out through the inlet may not even work due to the head design. The motor control circuit would also need to be rewired as they won't spin backwards by reversing polarity.
Just fill & flush through the pump out... Each fall before haul out I fill our tank with a strong bleach & water mixture and let it sit for about a week on the mooring. We have a valve on the outlet so the bleach is not sitting in the macerator just in the plastic tank. I then pump it out and re-fill with fresh to do a final rinse. After a week of the bleach mixture sloshing in there the tank is spotless and smells like a hospital..
One trick with macerators is to keep them mounted just above the level of the tank fluid and have a valve between the tank and macerator inlet.. The acids in the tank can do a number on the macerator. Once ours is drained out the valve is closed and the macerator sits dry. When we need to drain the tank the valve is opened and the macerator pumps it out.
Our Jabsco macerator is about 18 years old, I have the receipt from the PO, and it is still going strong but it always sits dry without black water in it......
This is what the inside of our 33 year old holding tank looks like at the end of each season before winter storage... The dark spot near the top is the tank label on the outside of the tank. The hole at the bottom is the tank outlet that goes to the pump out and macerator.
I have never physically scrubbed it just blasted the insides with a hose and used the bleach/water trick each fall...
If you can valve it off and used the bleach/water trick I highly recommend it...