Is there a way to mark up text documents from DT using an app such as Textilus?
You can open any document from DEVONthink into any application on your computer that supports the document’s file type. Use Data > Open With… or choose that command from the contextual menu or add the Open in External Editor or Viewer icon to your toolbar. DEVONthink doesn’t care what the external app does with your data.
Textilus is an iPad app the last time I looked. If you can Open In … from DTTG and Textilus is listed, then you can use the app to edit. The problem is getting the edited file back into DTTG.