Well I spent six hours going down a rabbit hole to figure out a work around. I thought maybe I could use their "web box" display item to load some Javascript to manipulate the page, but that lead to issues with crossdomain blocks, so in trying to figure out how to host an html file on the Sharepoint server, even after created a new content-type that should identify an .html file as text/html, instead it keeps sending as binary so that instead of viewing it triggers a download.
Via a "real" SharePoint server it would be possible to set both "true" mime-types, or set the "BrowserFileHandling" property to passive; but after loading up all the necessary PowerShell addons to connect to a web based SharePoint server, they've even made this property "read only" for the 365 version of SharePoint.