The correct tub that has the required integral tiling flange will not have room for a deck mount anything.

You would also be asking a contractor to drill a hole in your new tub where one is not intended to be. I bet a lot of contractors would refuse to do that. Putting a hole there in a fiberglass tub may ruin the integrity of the tub structure. Drilling through a porcelain and cast iron or even porcelain and steel one is way beyond the capabilities of most.

I can understand why you would want this, but you might have to give up on the idea. We have retractable hoses under kitchen sinks, because there is access to them if they might start to leak. This would not be so under a tub like yours. Having used a bidet hose, I know that the water from the sprayer likes to flow down the metal hose and drip onto the floor from the bottom of the loop where the hose hangs down. Again, this would be a point for rot under a tub, where one could not clean it up.

I'm not sure I understand the problem. But, is this the solution? https://www.homedepot.com/p/Pfister-Deckard-Single-Handle-Deck-Mount-Roman-Tub-Faucet-with-Handshower-in-Brushed-Nickel-RT6-2DAK/305095143

There are other styles on the home depot page.

A lot of different retractable shower hoses/heads to choose from. They come in either wall or tub mounted.

Google Tub mounted shower or retractable wall mount shower. .. .. Then go to town with your research.

@ fuzzy wuzzy - the issue is the tub the OP is selecting. Unless they buy a tub designed for a roman tub faucet hand held being installed into it and then have the proper diverters and air gap pieces as well as access to the hook ups underneath - there i no way to install that into a regular tub.

The faucets are easy to find the proper tub and execution is the issue.

HU, no matter where such a handshower would be placed, you would need a water line traveling to a new place in your shower, so why not simply get a wall-hung hand shower and have it placed on the wall? If you don't want the hose in the tub, then hang it on the wall just to the side of the tub. Just be sure to put an on/off valve on it. I put one like this next to my toilet in the half bath, with a temperature control valve also on it. I now have a warm water bidet rinsing hose. I also use it to fill up my mop bucket.

https://www.sanicare.com/Bidet-Mixing-Valves-s/89.htm

As others have posted, just have an additional hand held installed wherever you like.

I have two hand held showers in my shower - one is by the standard shower head and one is at the other end by the bench.

The one that is by the bench is mounted at the optimum height to access when I am sitting on the bench. There is no reason that you couldn't mount the second or third hand held at whatever height you want.

I am located in California where one can only have one water fixture going at the same time so I have a three way diverter which changes which fixture water is coming out of.