It's unfortunate that there isn't a general Udacity forum for questions like this that aren't directly related to CS373. :)

My guess is that for now, the best you can do is to accept dropping the assignment. I think you have a valid question, but I suspect that it's not a high priority given the technical challenges Udacity already faces in trying to figure out the best way to provide us with classes and grading. Figuring out the logistics of turning things in late is a bit of a challenge given that the solutions are available in videos, and are discussed in the forums, as soon as the HW deadline has passed. As a result, it's obviously unfair for anyone to turn in the HW late if they have access to these materials.

The only solution I can think of at the moment would be to have a flag on your account, which lets you put yourself into a "I'm turning things in late" state, in which you don't have access to the forums or solution videos until you leave that state. While you're in that state, you can work on old homework assignments, but as soon as you leave that state, you can't change old homeworks. This is non-trivial to implement given the current separation between the forums and the main site, and it also may not be the best solution considering I just came up with it in the past 5 minutes. :)

Finally, I get the sense that most of these classes will likely be offered on an ongoing basis, so your other option is to wait until the next iteration of the class so that you can be in town for the entirety of it, which I admit isn't a particularly satisfying solution...

tl;dr version: It's a valid problem, but hard to solve, and likely won't be solved anytime soon. Of course, I'm not employed by Udacity, so take that statement with a grain of salt. :)

Its certainly a great question. I am in the same boat, I have a gazillion things going on in my life, work, classes with Udacity, experimenting with classes at Coursera, (not nearly as good lol), I also work overtime at my work so... its very tough, and I really want to continue on schedule though. I would hate to 'drop out'.. :-/