Backerboard under Fiberglass Shower Pan?

I am about to install a MUSTEE 32x32 Fiberglass shower pan for my shower stall in master bath.

I am also putting in new ceramic tile while I'm at it and I am going to be putting backerboard (Durock or Hardy) to do so. Since that will raise the floor 5/8 inch, I am considering installing this under the shower pan mainly so that there is no seam between where the shower pan and the ceramic tile will meet (paranoid since this is where the rotting was that caused this replacement). So is there anything intrinsically wrong with putting Durock or Hardy backer cement board under a fiberglass shower pan like this?

As an aside to this, the instructions do not indicate any means by which you should secure the shower pan in the alcove. They indicate use of clips for a wall surround but not if you are using ceramic tile. The previous shower pan had some large head nails securing it in place above the flange (obviously not through it). Am I to assume that since the cement board will overlap the flange will be enough to secure the shower pan?

 

Shower pans are usually secured to the studs in its perimeter by drilling a hole halfway up the flange and using a roofing nail or screw. Your cbu will overlap the flange leaving it a little short and overlay your tile to completion. That will keep the cbu from wicking water.