I've looked into dedicated hosting services in the past as I was interested in such a thing. It seemed like many services (those that tailor specifically to low latency game servers) aren't interested in hosting small-time indie games like Blockland.
Your specific idea does not appeal to me, however. I'd rather not donate money to pay for awful DMs, fort wars, and a bunch of custom add_ons. A better idea would be to pay for a single physical server, and run multiple BL servers off it. All you do then is divide the cost of the server between the number of BL servers being hosted and those that donate end up paying a certain amount per month for their own personal server running whatever add_ons and game modes they like.
There are other issues too. Where would this server be located? It isn't really fair to host it in England/Europe if you want American players to participate. Also there is the issue of who has what rights/privileges over the server and its operation.
It's a cool idea, but it requires a little more thought and explanation than what has been provided thus far.