Some Oxbridge dons believe it could give able students a better grounding in academic subjects than traditional A-levels.
Last week independent school heads rejected changes proposed by a government taskforce.
Mike Tomlinson, the taskforce's chairman, presented the proposals to admissions tutors from Cambridge university this month.
One of the diploma's aims is to improve the breadth of young people's education, although critics believe it would reduce the depth of study in individual subjects.
Mr Tomlinson denied this. He said: "I am in the business of dumbing up, not dumbing down."
He said students would be able to pursue supplementary courses to give depth to traditional A-level-style study. For example, a sixth- former studying history might take a language course to help them read foreign documents.
The taskforce's plans were presented in outline form in the summer. Full details will not be finalised until next July.
However, Dr Geoff Parks, director of admissions for Cambridge, said: "We had a very constructive meeting with Mr Tomlinson. We are very encouraged by his ideas."
Dr Richard Barnes, admissions tutor at Emmanuel College, said: "We like many of his ideas."
Tutors welcomed the proposal for supplementary subjects. They said it could help prevent high-flying sixth-formers trying to accumulate high grades in many A-levels, instead of studying three subjects in depth.
The Qualifications and Curriculum Authority, in its official submission, highlighted potential problems for Mr Tomlinson's group. It said that the complex infrastructure of schools, colleges and training providers had not been designed to support a single learning phase for 14 to 19-year-olds, one of the aims of the report.
And the taskforce's suggestion of cutting the number of exams and placing more emphasis on teacher assessment would not necessarily be welcomed by teachers, said the submission.
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