Ignoring the advisability or not, the questions seems to be 'Is it legally possible?'
I think yes. You're flying on an SPL, not PPL, so the SEP/MEP provisions don't apply do they?
My understanding of UK/JAR provisions:
You aren't given an SEP prior to your first solo. You're given an authorisation by your instructor to do that specific flight. Exactly the same for the multi solo.
To gain the PPL the student would have to meet all the hour requirements for the licence + the MEP prior to applying.
Not sure if the PPL test could be done after the PPL hours are met but before the MEP minimums.
We all seem to concur that it's a less efficient way to proceed with vastly increased cost stemming from greater $$$/hr, increased training times due to the complexity & speed.
As for increased danger: Why should that be the case? It's the job of the instructor to ensure an acceptable standard is reached by the student before sending him/her solo. That includes handling system failures. It would take longer than for a single, but need not be any less safe.