This is the sort of situation which I am on my guard with.
It happens in the UK as well in the none NATs fields.
Teeside is an example where you will be cleared for the procedure and won't hear anything until swapped to tower.
It all boils down to situational awareness, you should know where you are at all times through the vectoring procedure because you are still responcable for missing the ground. Which is why thank goodness there are still so many NDB's on the airfield. A combination of RMI and DME from the field combined with the heading the controller has got you on should tell you when to start asking. ie if they put you on an intercept heading and your not at platform you start asking for it. They may at that point been intending to leave you there. But after you ask they then know what you want and generally get it.
Problems do occur when either they try decend you to below the platform alt or their radar vector minimas don't go as low as the platform height.
The desent below platform is a bit dodgy because in the event of a coms failure you are on a potentially none terrian safe vector potentially with not alot of track miles to sort it out. You just have to climb to MSA and then head towards the standard miss approach and do all the other coms failure stuff. It wouldn' be very nice.
If the radar vector minima don't allow you to decend you have to know what the protected area is of the approach you are doing. I use the out bound limit of the procedure ie if the plate has you beacon out bound to 8 D once your etablished on your inbound track and passed 8D you can then decend to your platform alt. In some cases the platform alt for a A,B class aircraft is lower than the C,D approach. In that case just use the C,D platform alt and its top of decent. There is some debate that if you do that you then need to use the C,D approach minima. I don't, I use the B for my aircraft, I can't see why your minima should change when there is no difference to the approach below 1000ft.
But as all these things its not a fight with ATC. The sooner they know what your requirments are the less stress for both partys. So request it before it becomes and issue ie before they put you on the downwind with 200knts ground speed. Intially they might sound suprised because they have been vectoring big tin all morning which can work it all out with a couple of buttons. But I can't imagine many who would get the hump.
The only hassels I have had is when its been a change in runway causing a major workload in the cockpit and on the ground. By the time the plate have been swapped and the brief started you are already late down wind.
Normally I just put the request after the intial booking in when they tell you the approach to expect. eg "Radar Vectors NDB/DME RW 08 request platform alt before decent with the procedure"