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Guys,
I would never use Hardie Backer boards as my base to do an interior floor. For the same reason a good tile setter would never allow his tile layout to put a grout line at a seam of the underlayment. The grout will crack and fall out with time as the floor moves due to flex. The only way that I would try it is if I was going to put a crack isolation membrane over the surface with fiberglass but with that there would be no reason for the Hardie Board.
The way that I have been doing this for the past 15 years (and has not ever failed) would be to put down roofing felt and expanded steel / lathe and then fill it with ThinFinish and / or TexturePave. Although, You will still have the slight possibility of hairline cracks because it is a concrete product.

Step 1 – remove all floor coverings to get back to the ¾” plywood. If the home is older and has 1x6 or 1x8 planks then make sure they are attached well and install a sheet of plywood over the surface which is glued and screwed down.

Step 2 – Install roofing felt by stapling down and making sure there are not wrinkles.

Step 3 – Install wire lath to the surface by stapling every 4” and overlap all seams by 2”. If you don’t overlap the seams you will have cracks that outline the lath.

Step 4 – Fill the lath in with either ThinFinish or TexturePave. If the floor has more flex I use TF but I prefer to use TP when I can. Once the surface is dry you will need to rub brick it and apply ThinFinish over the top to make it smoother.

There are other methods that guys have been trying and so far have been successful but they aren’t that old and they do cost more in materials to install then the time tested method does. If it aint broke don’t fix it!

Applying sand to rejection over an epoxy base coat works if you are going to apply a cementitious overlay like ThinFinish if you are trying to block stains, vapor transmission (using VB5) or if you are going over a sound flooring system that has been properly prepared. An epoxy floor coating such as UV1, PT1 or PT4 will NOT stop joints from reflecting threw the surface. In fact by adding the broadcast sand you will make it more rigid. You can try to minimize that reflection by using a fiber glass screening made for that but they not designed for use with these epoxy coating. The proper way would be using an epoxy or urethane crack isolation membrane over the surface at a minimum of 60 mils with the fiber glass screening. The cost of that system would be more than $3.00 per sf you have estimated for TP and lath. The other issue is that these materials are designed to flex so you won’t be able to put the epoxy base coat and the reflector coat down to thin because of the possibility of compression damage.
The fact is that if you have a customer that wants these floors put down over an interior wooden floor they need to pay for what they want. I wouldn’t take the risk of trying something new on a paying customer’s floor. We all know who is going to get the call to fix any cracks even after you telling them the best way to do the install but decided on the cheaper method.