Stay with me ..... Instead of presenting the user with a form filled with all the options that are available, you could present the process in the form of a questionaire about the site or idea, then using your knowledge of free hosting you could suggest to them one or more plans which they can choose from.
I dont think you'll be able to stop people saying I can offer more than that, only I don't see it as a problem, if the process that has decided what the user should request is a reasonable one, and users are happy with the suggestions they are getting, then what the host thinks on the other end is, well, void.
People would come to know how to "abuse" the system, but then if the maximum anyone can ever ask for is a reasonable amount ( remembering this is free hosting, and providers that offer the world last months at most ) then even the largest requests won't be an issue, will they ?
I suspect there will always be a few people that don't agree with the process, but then there will be far more that are grateful of the help, and all the hosts here would never be able to complain again that free hosting requests are ridiculous, the formula that you suggest plans with could be revisable, it wouldn't have to be a restriction in the real sense of the word, more of a feature of FWS to better inform users about how to get the most out of the hosters that hang about here, and in the process eradicate ridiculous and unreasonable requests.
I think maybe if you made a post outlining the idea from both a hostee's and a hosts perspective, and held a simple "do it or not" poll, you'll soon have a pretty good idea about wether it will work in the long term, from your users perspective at least.