Reddit prevents you from doing this; I believe if you try to upvote from an alt account it wont work unless you're doing it from a different IP address. But, I just created an alt and successfully upvoted my own comment (I know there are better ways to spend a Monday morning - I'm bored and putting off work).

Not sure if this is fixable or if the solution would create any privacy concerns. But, it's something to be aware of; this community is still small enough that a handful of upvotes can move a post up the ladder, so abuse is a very real possibility.