Craig,

Thanks.  You seemed to miss my point that MUST, which you take as BE SUPPOSED TO, has another periphrasis as HAVE TO.  What you appear to say is that OUGHT TO is not an infinitive form, so can't be preceeded by the HAVE TO.  It is true that the TO is missing.  I think we must admit the meaning of HAVE in this context is not tense, but a subjective MUST with the meaning of obligation.  The HAVE is in a past tense form, that with modals does suggest a smaller degree of probability or certainty in this subjective use.  I see now that what you meant when you said "colloquial," was that the phrase is in a class with "shouldn't ought to do it."   I thought that you were thinking of such phrases as "hadn't have done it," which contain the past perfect tense form as the subjunctive thus meaning "wouldn't have done it."   These are far from just colloquial.  

Bruce

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