When you can just plug in a USB drive I think you are wasting the TC.. and power.. and lifespan of the Mac.
It is certainly difficult if you have various different user permissions on the synology..
Really you should use the TC for TM.. if you don't then you don't need it.. I would sell it off on ebay as it is causing you to dream up ways to use it.. some things are a burden to our minds... if we have it we have to use it.
I suffer from the same affliction.. but to overcome it sometimes.. you need to really think is this worthwhile.
Mind you, using a standard Mac backup program you can probably just do an incremental backup of the synology and it might not take very long at all.. if you are not loading huge masses of stuff in and out of the NAS.
Might be easier than getting rsync to work.
Sometimes the speed of copying in this manner is really poor though.. !!