Note: this is the opposite of backfeeding, for which there are many questions and answers. Have not seen anything on the internet about this scenario.
Say you had a 240V generator inlet with a 50A breaker professionally installed, and you needed to run a 240V appliance close to where that inlet is, but where there is no 240V outlet available. The hypothetical appliance is three-wire; the outlet is 4-wire/4-prong. Aside from the interlock/lockout kit on the breaker panel, is there anything dangerous or wrong with wiring the appliance with the matching generator plug and powering the appliance through the inlet?