Talking about the fitting of recessed lighting into an L&P ceiling, I used to get access from above, then clean the area (the plaster will not stick well if it's really dusty).
I would mix up bonding with a strong solution of PVA and reinforce the area round the site of the fitting. This would be left to go off for a good few days.
Then the hole would be carefully cut with a
very sharp holesaw. That's difficult, because they don't stay very sharp for very long after cutting a couple of holes through that stuff...
Finally the hole's edge would be sealed with some more bonding/ PVA mix.
Having said that, I have only ever installed recessed fittings into L&P ceilings on two jobs. It worked out far too expensive for most. They still had recessed lighting fitted, just done VERY cheaply.
After quoting one customer, I went back to do a socket. In the meantime, they had had someone else to do their recessed lighting.
It was awful. Oversized holes, with light spilling out, broken plaster and laths on virtually every hole (the bloke had told the customer he could not guarantee the integrity of the ceiling and that there would be "some" decoration to do afterwards...), holes cut way too close to joists, undersized secondary cables (huge volt drop....), no cpc continuity throughout, all connections choc-blocked with no further protection, must I go on?
The sad thing was, the lady was happy with the job. The really sad thing ws, the job was only 10% cheaper than my original quote.