You may not be aware of it, if your machines are localized to English, but there is a prominent example for doing just that: Microsoft. The way they do it might give you some ideas.
I live and work in Germany, and my work computer has a de-de localized Windows and a DeNIC IP. When I visit a Microsoft KB article, I automatically get a machine-translated version of the English article. I don't know if you can get to it without using a localized Windows and/or a foreign IP (although I hope that the IP will not be important). Here screenshots of what it looks like (yellow marks in the first pictures are mine):
The above is a picture of what I get to see when I visit the page. There is a disclaimer that I am reading a machine-created translation, and a link to a page which looks like it will contain some explanation/apology, but in fact contains an appeal to the users to improve the translation if they notice errors, with short help texts hidden behind a navigation menu. The link goes to http://support.microsoft.com/gp/machine-translation-corrections which I assume will be available independently of your localization.
I have no idea why the language selector is preset at United States (English) despite the page being displayed in German. I find this confusing. Actually opening it and clicking on United States again shows the actual article in English, but other parts of the page, such as the "Wichtiger Hinweis" (important notice) stay in German. I find this confusing too.
When I hover over a paragraph with my mouse, it gets highlighted, and I see a tooltip with the original text and a clear possibility to improve it. (The subdued yellow rectangle on the following screenshot is part of their UI, not something I marked).
These are some ideas how you can do it - if you decide to do it at all. It may be better to not offer a localization of inferior quality until you can provide a usable localization. I think that your target demographic can be split into roughly three groups: People who can and want to use the English original even if a good localization is available, people who could use English but prefer a localization to their native or everyday language, and people who do not know enough English to use a non-localized version.
I belong to the first group, and I can tell you that the availability of a machine translation offers me no benefit. If I can chose this version in the settings, I will never do so. If you preset it for me based on location or something else, I will probably go to the settings to change it to English, and on my way there will notice the quality of the translation and make unfavorable conclusions about the overall quality of the application.
The second group will probably not benefit from the machine translation either. They choose the localization because it is more comfortable for them to read swiftly in a language they know well than to try to parse a language they are not so good in. If you give them a machine translated localization, there is no option for swift, pleasant reading, they have to choose between two equally challenging options, one of which (machine translation) can cause resentment, because 1) it creates expectations it does not fulfill (which is the mechanism which ulitmately creates all dissatisfaction in consumers), and 2) nobody likes to see their native language mangled.
The third group, who does not speak English, will benefit from the presence of a machine translation - if they have to use the application at all. If you are a monopolist, this is a good thing to offer them. If you are trying to gather a user base and hope that localization will enable you to penetrate a foreign market, I don't think that machine translation will be of any use to you. As soon as you have a local competitor, the bad localization quality will be a large competetive advantage for him, and you will probably get bad reputation on this market.
So, depending on your goals, it may make more sense to schedule localizations for a later release, when there are ressources to do it properly. If you prefer to (or have to) do it now, no matter how bad the quality, Microsoft's example may help you with ideas.