Yes, use batten.No don`t use rawl plugs........mark where the batten`s going and then find the joists thru` the plaster by poking a bradawl thru`... cover the holes with the batten and long screws into the joist _thru batten+ lathand plaster+ about 1 inch into joist
there will be some size of joist, in the "mini-roof"----------you could invest in one of those stud+joist finders.like the buried cable finders If you really must go into the laths, then use a butterfly toggle between the laths trust my pshycotic eye
if youre running a batten along, then mark where it will go, and use a small nail and a hammer to tap through the plasterboard to find one. the next one should be at about 400mm from it. you may end up with a row of small holes but it will be hidden by the batten and you will get a decent fixing.
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