I've done a little bit of digging and it seems that SMPlayer will not remember the position of a file opened as a network stream. I tracked down this post about it.
There is a trivial solution for the situation in this post, simply changing file:///path/to/file to /path/to/file. However when using http, this is not an option. Is there any technical barrier to remembering the position of a stream? If not will this functionality be added?