I guess it is a sort of grey-area; however, some people on this forum have posted about getting notices from DH for improper use of their accounts (storing data not related to their website). Just something to be aware of.
My account claims to have "unlimited" disk space, but I think that has a lot of caveats attached to it.
Are you saying you have 1TB available in your plan, or that you currently have 1tb of data stored on your plan for the last 10 years?
I am not really sure if Torrentsync would be possible to get up and running on DH, but a bash-script with rsync is certainly do-able, and fairly simple to implement...
The one advantage that I can think of that Torrentsync would have over rsync is that Torrentsync can handle large files better in case of interuption during transfer.
I imagine there are some pros and cons to either. Might be worth looking into.