I have absolutely no idea if this might ever come to pass, but the subject certainly deserves some discussion.

Tito Mboweni is reported to have mentioned that the possibility of having a different interest rate base for mortgages has "entered his radar screen".
South African Reserve Bank Governor Tito Mboweni says the bank has had a strategy meeting at which the matter of delinking the mortgage system from prime interest rates was discussed.

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Mboweni was asked whether consideration had been given to a tiered increase in interest rates to stem inflation but boost spending on good debt such as home loans in particular.

Mboweni said South Africa had an inflation targeting framework where the central bank targeted inflation directly, but it was measured on CPIX - consumer price inflation minus mortgage costs.

"But maybe we didn't think through this thing very well at the beginning ... because whenever we implement monetary policy changes the first impact is on mortgages and we ... within that we should investigate the possibility of approaching the mortgage interest rates ... so that (the mortgage system) becomes independent of the volatility of prime interest rates."

Noting that there were "so many people" with no assets or property who now wanted to get into the South African property market, he said the volatility of interest rates "is not of assistance to them at all".

"That has entered my radar screen," he said, acknowledging that the bank had held a strategy meeting at the weekend at which the matter had been discussed.
full story from Fin24 here
Good idea, bad idea?

One thing that occurs to me is it might make non-mortgage rates more volatile.