Can you use Iron Leaves ex's "Rapid Vernier" Ability while Klefki's "Mischievous Lock" Ability is active?
No, you cannot. Rapid Vernier does not activate until Iron Leaves ex is on the Bench, at which time it is considered "in play" and therefore blocked by Mischievous Lock.
Source: TPCi Rules Team (2024-06-20)

Relevant cards

Note: All applicable cards may not be listed below.
SV5_25
SV5_25
SV1_96
SV1_96