The block off/by-pass plate that IPD sells is just the same as dremeling out the passage on the old non-functioning solenoid. This is to say that it will work just fine providing your problem is a dead solenoid and not something gone wrong inside of the box.
Yes, fixing it properly is the best way to go about it but have you priced factory solenoids lately? They list for about $300.00 and my cost on one is around $200.00, pretty spendy in my book. The first thing you have to ask yourself is when was the last time that you actually used the button on the side of the shifter to drop it into or out of overdrive? If the answer is almost never like most of my customers then I would advise you to save the money and just purchase the IPD part or pick up a used one at the bone yard and dremel it yourself and be done with it. I live and work in Orange County, So. Cal and it is pretty flat here so my customers don't even know that there is a button on the side of their shifter. Should you live in hilly country then I would advise you to bite the bullet and fix it right. Should you decide to fix it right then spend the money and get a factory solenoid or one from Nippondenso, same stuff. They cost almost twice what the aftermarket garbage does but the difference is night and day, spend your money once and fix it right.

Mark