For the last few days I have been thinking about whether to integrate Joel's code into the calibre driver. I have not been complete convinced it was a good idea as Kobo do not really support this. Also, it isn't something I personally need. While I do buy books from Kobo, I generally read the sideloaded ePub version. A lot of that is because I do fiddle with the formatting to get them how I want to read them.
But, after reading a few things and trying a couple of others, I have decided to do it. One of the big reasons to do it is the support for RTL languages. Another is ePub3 support. Kobo seem to be moving ahead with this, and at the moment this is the only way to use it for sideloaded books.
Having said that, don't hold your breath. It won't be done this week. The changes aren't that hard, but I need to test them to make sure they work, don't break anything else and aren't a performance problem. Plus, if I find a problem with this, I might stop. And other things, fixing bugs in the driver, work, family, reading, sleep (not necessarily in that order) will take precedence.