Fitting a career framework around a nationally negotiated pay spine: worth attempting?

I work in HR at a UK university with about 2,900 professional services staff. Pay is set by a nationally negotiated spine and our grades are determined by a long-established analytical job evaluation scheme, so the levels themselves are not mine to redesign.

Staff survey results say people cannot see how to progress, and 'progression' is our lowest-scoring item at 31% favourable. I have been asked to build career pathways, but the only real lever is applying for a higher-graded vacancy.

Is a career framework credible when it cannot change how anyone is graded or paid? I would value hearing from anyone who has built one on top of a fixed structure.