Yes, indeed, some hybrids have the 12V battery in the trunk. Yet, this shouldn't matter, as the voltage is wired everywhere to the car, so it should be reasonably easy to arrange a socket anywhere. Remember the fuse for the socket, as Solar Mike said! Usually, there is also a jump start connector under the hood, so if you don't want to install a new socket, you can connect it to this jump start connector and any metallic grounded part. However, the jump start connector may be unfused, so be careful. In conventional cars the jump start connector is definitely unfused, in Prius I don't know as Prius jump start is just a computer boot-up.
The 12V battery in Prius is not a starting battery. Thus, you cannot expect to draw hundreds of amperes (well, hundred may be okay, two hundred a borderline, three hundred I wouldn't draw). However, 20A will absolutely not be a problem. It's only about 250 watts, far less than what the HV-12V DC-DC converter provides in the Prius. You can draw 20A with the hybrid system on or off, if it's off then drawing 20A for multiple hours continuously will drain the 12V battery. If it's on then it will be recharged with gasoline.
I don't think you plan to fill tires for multiple hours, so the battery current and capacity requirements are satisfied.
By the way, have you considered a bicycle track pump? If the tires are not completely empty of air, you can with little effort fill them to the ideal pressure with a track pump, no electricity needed. A good pump doesn't require massive effort for that. Increasing pressure from 2.2 bar to 2.5 bar on all four tires doesn't even make you sweat.
What Solar Mike said about engine size and capacity doesn't apply to hybrids, as the internal combustion engine is started by a high-voltage battery. Thus, a 1.8 liter engine in Prius has smaller 12V battery than a 1.8 liter engine elsewhere.