The only joysticks you can switch from 4-way to 8-way without opening the control panel are the Mag-Stik and the Omni-Stick Prodigy. Or the T-Stik Plus, which are discontinued, but you may find somebody selling a set. The Mag-Stik (and the T-Stik Plus) switch by pulling up on the handle and twisting, the Prodigy switches by moving a lever that would come through your control panel.
If you don't mind opening the game to switch modes, The Omni-Stick (no Prodigy), Mag-Stik(no plus), T-Stik (no plus), E-Stik, Euro-Stick, and J-Stik all switch easily, just rotate the restrictor plate. The E-Stick might be the easiest to add to your Nintendo control panels, due to it's unique mounting method.
MCA Australia, Sanwa, Semitsu, and most other Asian sticks have unscrewable restrictor plates. With these, you have to use a screwdriver to switch modes.
Wico, Happ, Betson, and IL joysticks don't use switchable restrictor plates, they just have switchable actuators. Basically, forget these as switchable sticks. Changing the actuator to 4-way mode makes it impossible (or just very hard) to hit a diagonal, but leaves large dead spaces in the joystick, and won't have the feel of a true 4-way. The results will not make you happy. These are good sticks if you're building a frankenpanel and want dedicated 8-way and 4-way sticks, but on a Nintendo coctail, you're not building a frankenpanel.
Last, you could go with a 49-way joystick and a GP-Wiz49. This combo will switch modes electroncally, or at the push of a button. There is no physical restriction of the stick, so you won't have the feel of a true 4-way, but the higher resolution of the stick eliminates the dead space problem you get with the Happ-type sticks, and will yield pretty good results in gameplay. Another bonus to this option is that in addition to 4-way and 8-way mode, the sticks have a 2-way vertical mode, 2-way horizontal mode and a diagonal mode for Q-bert. They also will do a good job on games that originally used a 49-way stick, Hall-Effect Stick, and some analog stick games.