In Street Fighter IV you can remap the Select key to be easily accessible so you can plink your light punch.
Is this "trick" legal or illegal on major tournaments?
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Generally, it is allowed.
The latest version of the Evo 2014 ruleset permits custom controllers and sticks, including Hitboxes and sticks with custom button layouts (including sticks with a Back/Select button on the top surface of the stick). The only things that are explicitly forbidden are wireless controllers, and the use of turbo functionality.
Most major regional tournaments in North America generally follow the Evo rules as well. For minor local tournaments and foreign tournaments, you'll need to ask the tournament organizer.