Your idea sounds basically right to me, although I would suggest a slightly different approach so you don't run into problems when trying to upgrade.
Instead of modifying the code itself, you could write a plugin that lets you achieve the same result. You'd have to register against the LoadHandler class and 'intercept' an article page view. In there, you can check if the galley type is a Flash type (you wouldn't have a method, but you can check by looking at the file type) and if it is, you can then serve the page (you'd have a "FlashArticleHandler" or something of this sort in your plugin).
Be mindful of whether your LoadHandler callback returns "true" or "false", depending on whether it found a flash galley type or not. You'll see what I mean with this when you start your coding.
You can look at the xmlGalley plugin (in the generic category) as an example. It is slightly more complex than what you'll be doing, but if I remember correctly, it has all the right components to guide you.