Not THAT hard. I've written a couple of sequencers on Pd. And there are several out there.
It would be pretty/fairly hard. Softwerk has a huge amount of inter-lane modulation/interaction. The pitches, duration, velocities and gating of any number of lanes can be controlled by any other number of lanes. The gating concept also requires (i think) a fairly novel way of dealing with time that I think is hard to do in Pd. I could easily be wrong.
I suspect it would be the 80/20 or 90/10 rule ... easy to do the first 80-90% .. hard to get the whole thing. And Softwerk goes significantly beyond what can be done with the Schaltwerk hardware that inspired it.
I never released it because it is still missing the one key feature that I would want: the ability to sequence not just the internals of patterns, but different patterns themselves.