For using seq24 (or other software) inside Zynthian(MOD-UI) this plugin has to be LV2 compatible. Hmmmm - as I wrote this I think I have to explain this a little bit:
Zynthian can handle nearly all kinds of plugin. But there are limitations due to the license, the integration of the user interface and maybe also the plugin type (and of course the availability of the source code because I think there are no binary plugins around which are compiled for ARM/Raspi).
Some plugins have a direct jack interface, some VST, some JUCE, some LV2, DSSI, … For me LV2 is a really nice and open standard for plugins and with mod-host as a plugin-host and MOD-UI as a universal UI for LV2 plugins the possibility of making music with open source software is grown much.
But now the limitations start: If a software is not supporting the LV2 standard, you have to add code for this by yourself (or find someone who will try this). For this you must learn how the plugin generates audio data. This is not really difficult (I have done this for Dexed) but you need to have some experience in programing and you have to learn how LV2 works.
The next “problem” is: MOD-UI has a nice web interface. So using every plugin-UI from the plugin itself (mostly based on X11/Qt/GTK/…) can be dropped. You have to arrange the UI with capabilities of LV2 which MOD-UI can use. For example a current limitation is loading data which is not representable in LV2/MOD-UI - like samples or sequencer patterns of different length.
Coming back to seq24: As I read it is not LV2 compatible. So the whole list of things for getting it into MOD-UI plus an idea how to integrate a flexible UI are on the to-do-list. But perhaps there are other ideas how to integrate it? I can only say: If you want a feature: try do implement it by yourself (with the help of others). I have done this and I must say there are really nice guys around who are willing to answer questions. Also: if you know how to do this you can use the knowledge for further projects.