If your site has serious accessibility problems, explain them. Typically, you would write a page about them and link to it on the main page. It might perhaps best be labeled “inaccessibility statement,” but in practice you probably want something like “accessibility issues of this site.” You might even use “accessibility info,” though this would be euphemistic.
In addition to describing essential problems, the page should help people to cope with them. Let us suppose that a large site lacks all alt attributes for images. This is serious indeed. However, it may take a long time to fix it, perhaps a year. Authors need to study each image and write the best alt attributes. Attempts to speed this up by using some bulk processing probably causes serious problems, perhaps even more serious than the one you are trying to solve. So what will you do? The proper way is to write and publish a description of the problem first. Here’s a suggestion, which might be somewhat too honest: “Unfortunately, most images on our site lack textual alternatives (alt attributes). This makes much of the content difficult to access or inaccessible to people who cannot see the images. We apologize for the situation. We are in the process of adding textual alternatives, and we expect this to be completed by April 1, 2009.”
Technologically oriented people are often inclined into looking for solutions rather than workarounds or explanations. Yet, when serious problems are detected on web pages (in accessibility or otherwise, e.g. in correctness of information), the first question should be: how do we minimize the damage? This typically means that you immediately add a note that mentions the problem and perhaps suggests a workaround, then start working on a solution. It may take a long time before a solution has been found and implemented.
Think about users and how to help them immediately, before considering how to solve the problem.