If you just want to rsync to it, you can run it over ssh.

You can also browse files over SSH, even mount it permanently. For just moving files around, it's a bit slower and resource intensive than plain samba or NFS but for running rsyncd, it should be fine.

NFS isn't compiled into the dd-wrt kernel so if you want to use it, you need to use the userspace version which is somewhat slower.

Routers don't tend to be the best fileservers. They have a great networking stack but the interface between the router and the drive is pretty slow. On top of that CPU overhead that wouldn't be noticed on a proper server can cause a little router quite a lot of stress.

There are also possible security ramifications of running a precious service like a fileserver on something with a direct connection to the wild.

A NAS (Network Attached Storage) device would probably give you better performance and not cost all that much. A lot of the QNAP devices are multi-protocol aware and perform quite well.

Sharing files with a dd-wrt router
I recently opened a container of store-bought dried apricots and they smell vaguely of fermentation/alcohol.  I've never noticed this before.

They taste fine, but should I be concerned?  Do they need to be used soon?

Worst-case, would I end up sick, or just tipsy? 
