Yes, that's what I'm saying.I cannot see how 'my' ramp test cannot work. ... Are we saying - not that it wouldn't/doesn't work but that it is unlikely to be useful because a circuit leakage of between the relevant values is unlikely?
It could, of course, be that a small leak (say due to water ingress), not itself enough to trip an RCD, was facilitating 'nuisance trips' when other things which also had leaks were connected to the circuit, therefore resulting in a 'totting' up of leaks to enough to cause the RCD to operate. In that situation, you might be called to the site, and your ramp tests ought to 'work' as you expect. However, if there are not enough such 'other leaky loads', then the primary sub-30mA leak is not going to cause any RCD trips, so you won't get called - and that is why I suspect that you won't often find yourself investigating these sub-30mA leaks.
I agree with the rest of what you say about how ramp tests ought to (and, I presume, do) behave in these circumstances - I just feel that, as above, you probably won't all that often find yourself investigating such situations.
There is, of course, one potential complication, although not one you're likely to see in relation to water ingress. If the primary fault is a N-E fault/'leak' that will presumably work 'in the opposite direction' to the test current applied by your meter, and therefore should increase the required trip current you measure with your ramp test.
Kind Regards, John