The ReadyNAS uses a debian based Linux OS if I recall correctly. It also has a built-in A/V scanner for write operations to CIFS shares:
http://kb.netgear.com/app/answers/detail/a_id/23068/related/1
That said, anything is possible. I was about to expound on the unlikeliness of this happening, but I've learned that new threats come along all the time, whether they are using mapped drives from the clients or random port scans on the network to find potential targets.
You really have to base your question off of risk vs. reward. Are these machines so critical to you that a single storage of the backups is sufficient or should the ReadyNAS offload known/good images monthly/weekly/etc. as needed? I would presume since you are using a ReadyNAS period for backups they aren't enterprise critical enough to warrant an expensive setup, so I think if you simply do a remote a/v scan from time to time of the NAS shares and document what you've done, that's about the best that can be hoped for.
You may consider making one off known/good backups from time to time and storing them separate if you are super paranoid, but again it's all about what level of risk you want to take and the likelihood (slim) that such a device would get infected by a Windows virus that hit a client.