I wonder if it might be "precision approach guidance" that is needed rather than 'visual slope guidance'.'
Hopefully quoting safetypee's useful contribution from a discussion on steep approaches is in context
The original ‘jet’ certification at LCY required ‘precision’ approach guidance, being either ILS GS or PAPI; thus if the ILS is u/s, PAPI is mandatory.
Cheers
Overrun