The floating well has the potential for many problems, I have listed several here. If any problem can not be mitigated, the floating well switch can not be used (I would not guarantee that all of the potential problems are even listed here):
1- Potential for latch-up. The N-P-N formed by the structure is half of an SCR latch. By having the floating well, you introduce the possibility of triggering the SCR and causing latch-up. Depending upon your power source, this could be a catastrophic event, damaging the chip and potentially even damaging the power supply or test equipment. There are techniques to mitigate this in the literature, but I am not sure if any of these techniques work for the floating well.
2- Varying leakage condition. Because the well voltage is not defined in the off state, there is a strong potential for the diode (D1) to be slightly forward biased, leading to a leakage from the capacitor that is sometimes present and sometimes absent.
3- Capacitive coupling. The capacitance of the N+/Pwell diode from the source to the well can allow signal to AC couple from Vin to the well. The N+/Pwell diode from the drain to the well can then couple from the well to Vout, providing a direct path for AC signal from Vin to Vout. If Vin is changing when Vout is holding, this can cause problems.
4- Channel charge injection. It is likely that the well node will be higher impedance than the Vout node, which means that much of the channel charge for the secondary switch will be directed to Vout.
5- If the Pwell becomes a higher voltage than the epi voltage (due to charge injection from Vin, for example), there will be a biploar with well as the base, epi as the emitter and N+ drain as the collector. The collector current will run from the Vout voltage, and can pull considerable charge from Vout without taking much charge from the well due to the beta multiplication of the bipolar, leading to the wrong voltage on Vout.
Due to the amount of issues with the floating well, I would strongly advise against using it without very careful consideration of all of the potential issues that the floating well could produce