What would a laptop's reaction be to getting 5V fed into its 20V DC barrel charge port?

If the laptop supports PD, then chances are it has a PD charge controller integrated circuit.

Connecting 5V to the barrel connector itself will probably do nothing. Likely it has diodes preventing flowback to the power supply, to prevent it back-powering the supply and wasting battery power when the charger is switched off. Other than that the 5V will do nothing as far as the laptop is concerned, as it is not enough to push any real current into a battery that will have a higher voltage than it does.

If that "decoy" controller is well designed then it will be able to function at 5V in order to do the power negotiation, and will then negotiate with the PD device to supply a higher voltage and current. Before that negotiation it will likely have a FET (or one on each line) that isolates the input from the battery until the PD controller enables power to go to the battery, which will mean that there is no chance of back-powering the 5V supply, and will make sure that the batteries only charge from a "good" supply.

If the power supply doesn't properly negotiate more than 5V then nothing at all will happen. There simply isn't enough power or a high enough voltage to charge the battery.