Creating a “sysex librarian” using the Renoise API is definitely possible. It’s just that no-one has done it.
This topic is about using Duplex as a Synth controller/Sysex librarian…I probably killed the thread with too much detail:
http://www.renoise.c…er-with-duplex/
The topic was probably inspired by this in the first place:
http://www.renoise.c…-out-and-sysex/
In short: it’s definitely possible to create panels for synths using Duplex. The level of detail that the panel designer in ctrlr has is simply not possible with the Renoise API (knobs looking like the hardware counterpart etc.), at least not yet.
You would want to create the “sysex librarian” as a separate tool though, as Duplex doesn’t care about which song is loaded (I think a good great sysex tool would automatically match presets to particular songs)