Prompt Cement, PVA?

[Manso]

Very much a newbie question.
I had a heavy brass/glass lampshade weighing about 10kgs which was removed while ceiling painted. Painter found it too tricky to replace - I agreed! I now want to reaffix that lampshade.
On further investigation I now discover that the ceiling is lath and plaster.
What I am now facing is a square hole which when cleaned up is about 4"x4" with some short broken laths projecting into the hole. Access from above is impossible and the space immediately above the hole, i.e. forming a loose "ceiling" to the hole, is a piece of MDF with a hole in it through which the power cable for the light runs. I cannot ascertain the size of the piece of MDF but it is loose- not so it moves around - but it can be substantially flexed. The distance from the true ceiling to the MDF is about 1.5"-2", i.e. the hole is about 1.5" -2" deep, the lath and plaster is just over 1" thick. I would much prefer not to hack back the plaster to find joists and set in wood with all the dirt and finishing that entails.
My thought was to cut away the projecting pieces of lath and plaster (reciprocating saw), brush out any loose plaster, seal with PVA then apply Prompt Cement to fill the entire volume round the flex. Finally, to reaffix the brass rose (3"x3") with rawlplugs and screws into the Prompt Cement.
Would this work in theory - is there a better filler (two part??) - are there additives I should put in the Prompt Cement to take the screws better?

 

[kingandy2nd]

Was the MDF holding the light fitting in place previously?

I'm not sure if your suggestion would work or not - the weight of the cement (along with the light filling) might be too much for the old ceiling to take.

If you're having difficulty fixing the light because of the movement in the MDF, could you not get some expanding foam to fill the gap to stop the MDF from moving?

 

[Manso]

The remark about the MDF was originally just to give the complete picture.
As it happens the reciprocating saw was a no-no - vibration was too great.
What I intend to do now is try to screw a 4"x4" "sandwich" of various thickness of ply/MDF to that piece of MDT -that's going to be tricky since there will be protruding bits of lath between the "sandwich" and the existing MDF. I will make up a number of layers for the sandwich, each 4"x4" but of various thicknesses, so that when screwed tight to the existing MDF it will sit flush or slightly recessed to the ceiling. Any shortfall from flush will be filled with e.g. two-part. Finally the light fitment will be screwed to the sandwich.
Regards