How about putting an omni antenna under floor in a duct. I don't think the floor has been laid yet. What would be the effect of concrete and screed. There would also be steel reinforcing.
Having followed the thread from the start, I agree with MikeML.
One idea I had was to use a direction finding technique using to antennas to determine if the transmitter was to the side or in front of the information point, but this would increase the receiver complexity quite a lot.
The idea of putting the antenna under the floor may work.
I have no experience of such an installation, but it will be a poor antenna, which is just what you need in a situation like this.
At this point, rather than trying to install an antenna in the concrete, install a "PVC water pipe" from the ideal antenna location to the receiver location.
If the floor has not been layed yet, the cost of doing this should be minimal.
In the PVC water pipe, install a "sleeve dipole" on the end of a suitable coax cable and connect to the receiver.
Why not get one of the transmitters and the receivers and make a test set-up of this and try it out.
A bit of practical experience is far more valuable that all the best guesses here on ETO.
JimB