I'm putting up timber stud partitions, and need to screw several top
plates into the loft joists in a lath and plaster ceiling (working
around various exposed features).

For a bunch of reasons, access from above is not so easy (not
impossible, just awkward).

My stud detector just bleeps everywhere (because of the laths and
nails).

The only thing I can think off is to drill a few test holes with a
fine drill (e.g 3mm) until I hit a loft joist. I reckon I could end up
doing a few of those for each fixing point, as the loft joists are not
very accurately spaced (Victorian building).

Anyone have any tips to make the job easier?

Where I have to fix top plates parallel to the loft joists (and no
doubt not directly underneath one) - I'm thinking to fix noggins above
(fortunately these will be in a more accessible area), and then use
120-130mm screws to be sure to bite into the loft joists.

Any other recommended solutions?