I figure I'll just give supporters a secret url. This also allows for an easy rss feed for supporters to use. I figure supporters will care about the secrecy of the url enough that the added convenience won't be a problem.
In addition to my IT job, I am also a risk manager. Managing expectations takes many forms, including the messages we impart - intentionally or unintentionally - to our audience. Sometimes, what we don't say says more about us than anything else. With that in mind, I want to offer a couple of thoughts on this idea from an IT "worst case scenario" point of view.
Note - these are *just observations* from an IT geek to help you make an informed decision. I have no vested interest in the material; I just think I would be remiss in my "IT-ness" if I failed to pass along these thoughts (not an exhaustive list, just food for thought).
-- Obfuscation should be confused with neither security nor secrecy. A secret URL will only remain a secret until a few of your "secret admirers" view it. Once they do, the URL will be captured somewhere - proxies, reverse proxies, http accelerators, etc. At that point, it won't be long until a web indexing service (a la Google) finds it and catalogs it. Setting content meta tags to prevent caching won't help either because some proxies ignore those headers.
-- By publishing a secret URL with no viewing cost, you *may* risk sending an unintended message to your donors that tells them a.) you don't place much value their contribution, and/or b.) you lack the technical competency and/or resources to adequately secure their interests.
Now, your visitors may not be sensitive to this stuff. This is the worst-case scenario, after all.

Unfortunately, I have seen some very "special people" cause a whole lot of grief over the most mundane of things. Best to handle them up-front, if at all possible.
Perhaps the simplest "fix" is to "manage the message." Let your donors know what their donation does - and does not - get them. This way, they can decide if they want to donate to get the content or seek it out via other means.
Personally, I think the Facebook option is kinda cool. For their donation, you friend request them via Facebook. Once they are friends, they get the premium content. You leverage Facebook's authentication mechanism (which means you don't have to build or manage one of your own) and let's face it - almost everyone has a Facebook page these days!
That's my $0.02 worth of free advice. Worth exactly what you paid for it; your milage may vary.

-cec