One feature that the 146/RJ has is a huge airbrake on the tail. This is deployed at glideslope capture and must be operational for an approach into LCY. On other aircraft e.g. EMB145 you could only use airbrake upto the first stage of flap.
It allows us to make an approach at approx 55%N1 caompared with 65%N1 for a 3 degree ILS.
I remeber reading somewhere on here that to certify an aircraft for steep approaches that the engine speeds have to a certain proportion of either idle thrust or normal approach thrust, but cannot remember. It makes sense though - you don't want to have idle thrust with 1000fpm fully configured.
Our a/c have steep approach buttons and I'm sure that all they do is desensitive the GPWS/EGPWS. They cost about £500 to install.