Can I get chain tension within reason, using half-links?
It depends. Chainstay length and cog and chainring tooth count all figure into this. If you have a penchant for trigonometry you could probably come up with an algorithm to figure this out by plugging in the appropriate numbers, but AFAIK there's no pre-made solution to determine this.
i have a "magic geared" singlespeed conversion. i love that particular bike in that configuration and keeping an eye on tension/stretch/replacement is worth it. just be aware of what you're getting into.
Derailleur hanger on frame?
If you're on a super tight budget you can cut up an old derailleur to act as a tensioner. Alternatively perhaps something like a Surly singleator.
If you don't like either of these alternatives as they muck up the clean SS chainline then these chainstay rollers are a bit more subtle: http://ecx.images-amazon.com/images/...500_AA300_.jpg