MongoDB: Is it possible add SECONDARY member with version 3.0 to a replica set where members have version 2.6?
This is basically the rolling upgrade procedure from the docs, with the only difference being that you will temporarily be going up to a 4 member replica set based on what you have described. Hence it's basically a viable strategy if you have concerns about 3.0. 

Couple of things I will note:

First, the priority zero step shows good caution but is not really gaining you anything. There is no realistic way for that node to become primary before it catches up (not considered up to date, and if the other two members fail there will not be enough votes to elect it primary with 2 out of 4). There's no harm in doing it, it just doesn't add much in a 4 member set.

The other thing that I will note is that the addition of a new set member is not actually needed. You can stop your current secondary and simply update its binaries to 3.0, restart it and it is good. Do the same for the arbiter, then step down the primary and repeat. No resync is then necessary because 3.0 is fine as a straight drop-in replacement for 2.6.