So, you can get Seq24 for Windows, but without a way to pipe it to PureData it's not much use. In Linux this seems to basically be done out of the box, although I offer you one warning: disable Jack support for Seq24, as this seems to result in what I'm calling the "Keyboard-Butt of Doom Crash" where Seq24 freezes in a loop and sprays MIDI messages at maximum bitrate until anything looking at the port has a heart attack. (Or disable ALSA, I think it might be you can have one but not the other...)
BUT, once you get past that, Seq24 is a totally indispensable companion for PureData. Seq24 will send MIDI messages with channels attached, allowing PureData to peel them apart from each other.
With this setup, I now have all the features I used to get with the ancient Voyetra tracker, except that the MIDI patches are all of my own design.
While PD can definitely do sequencing (and do a ton of tricks Seq24 could only dream of), you have to build your own piano rolls, which is not only tedious but prone to errors, and PD's selection of widgets is pretty limited unless you go out fishing for plugins. Seq24 fills a gap in the capabilities of the PureData + Ardour juggernaut that kept me from really jamming with loops. Loops in Seq24 aren't just easy, they're intrinsic to normal operation, IE effortless.
And as always, remember that MIDI messages coming into PureData don't need to be interpreted as simple notes. You can do anything with them, so your coded sequence in Seq24 might actually be tied to a channel dedicated to tweaking your synth or reverb settings. One patch and one stored Seq24 MIDI file could be a complete, nondestructive performance, and as I've discovered, you can pipe multiple audio channels to Ardour separately, which means Pure Data can be meticulously peeling each one of the instruments in your ensemble apart and pushing them through their own channels for Ardour to record separately.
Now THAT'S a Digital Audio Workflow.