It can be done with at fixed bleed. The bleed off of the wastegate line is of a fixed size. As boost increases the resistance of air traveling through the hole also increases. There's you taper. Couple this to a solenoid control and you can tune for pretty much any boost peak and any taper by varying the size of the bleed hole and the duty of the solenoid. If you are using the stock solenoid and have no control over its activity level, you can still induce a taper by varying the size or number of holes, however it's a lot easier if you can adjust the both the hole size and the solenoid duty.
Just for a picture of what's going on. Imagine blowing through a straw. Blow softly and air flows nicely, not very hard to blow. Now try to blow very hard through the straw, yes more air comes through, but you really have to push hard. See how that "tapers" as more CFM comes off of the turbo outlet.
The resistance to air flow is inversely proportional to the size of the hole and it increases exponentially. So, very tiny changes in the size, number, or holes has a huge effect.
I did this for awhile by poking holes in a bit of plastic tubing. I had a bunch of tubes with different sized holes or more than one hole. You figure out very quickly what works by just driving. One has to just be careful to overboost if the hole is too big.
Hope that helps.