If indeed there is no option, or it is a pain in the arse hack to do, then what about the network approach? Typically you are going to want a real router for this one, or maybe stick it through a PC/laptop which can fake it, but you might be able to get by on basic parental controls.
Figure out what the default homepage is, set the router to block that site and redirect it to your chosen homepage. Will not work if it is a local to the device page but if it does go out onto the wider network then you can do it, if it is an IP address rather than a website you might have other problems on a basic content filter for your router or using DNS but I doubt even Nintendo would hose it up bad enough that they hardcoded an IP into a browser.
Other problems will be if you actually want to go to the original homepage (hopefully it is not a shop or something you might want from time to time) but I suppose you could then find another means to do it.
Ideally I would hack it should it be necessary but if end results are what matters...