First be sure to save a backup copy of any file you plan to change.
What you need to do is look on your Kobo wifi in the .kobo/kobo directory for the conf file. Open it (it's a text file) and change all the website names to whatever start page you want. When you pick the URL, be sure to add a '?' to the end. You'll need the question mark to because the Kobo will add /home.html to your link and the question mark tells the browser to ignore it.
I would suggest that you pick the mobile Google homepage or that you find a way to make a homepage full of links. You might also decide to point the browser at the mobile version of one of the free ebook sites. If I were going to use this feature a lot, I'd probably have Feedbooks as my home page.
Given that you have to enter text with the d-pad I don't think this will be a very useful feature. Also, navigating around a page moves the cursor from one link to the next. This can take forever.
P.S. With a little refinement Kinstant could support the Kobo wifi. But I think it needs more customization first.