Currently use QC to make custom layers for Bonix TV - a vision mixing kind of application for OS X with a kind of QC stack of layers, each layer being a composition with published ports and some protocols for data types. Also generate sports graphics and twitter feeds and so on for video score boards at large sporting venues.
Spent a few years learning Quartz Composer as my way back into coding from a decade away doing design related work. Film, graphics, studied Architecture for a few years and got involved in publishing then film. Now moving into front side web development. Hope to get into app dev with Vuo and rapid app prototyping.
I recall there was some QC Core Image filter code in one of their example comps to do this.
Vuo doesn't have CI filters, so I guess converting from the QC CI round-corners patch (if somebody can find it, it's not in the patch library) to ISL shader code for a Vuo node is one way you could get this done quickly if you can code GLSL. I forget how the code went, but was pretty straight forward maths using a comparison operator with the radius input to knock pixels on the corners out to (r,g,b,a,) = (0,0,0,0).
But having programatic changing of input lists and ranges is something I'm really wanting was a constant source of frustration with QC for me, so I ended up making HTML?CSS/JS interface for one client doing livestreams at stadium venues.