SharePoint Online integration and executing Wisej.NET in an iframe

I’m evaluating the technical efforts to integrate Wisej.NET application as an alternative for extension and customization for Sharepoint online. In SharePoint online the only allowed form of customization is using javascript solution (call it extension)  running into the user browser. The javascript code is injected into the page generated by sharepoint and interact with the page and the page context. There is a sort of directory registration  for the extension that allow SHarePoint to manage the extension in terms of parameters,, The javascript extension are responsable for the creation of UI elements following the CSS used on the page.
Please note tha the domain for sharepoint online is always sharepoint.com. Avoiding iframes the Sharepoint extension not suffer from cross-site access problem.
Now come to Wisej.NET.  In the SharePoint on-premise environment is very easy to create a classic sharepoint webpart (an ASP.NET user control) that implement an iframe hosting the Wisej.NET. Having total control of domain and server code it is easy pass info as page context and authentication tokes from sharepoint to Wisej.NET, i’ve done this. In SharePoint online we can create iframe into the page structure but we have the problem that we can’t host our iframe source URL inside the sharepoint.com domain. SharePoint Online manage a list of domain for iframe url source. So we can create a client-side webpart that load the Wisej.NET application url and pass to the Wisej.NET parameters and context using a mix of rest-api/query string parameters/http headers. No real problem. The problems arise when we want to make interaction from Wisej.NET application AND the sharepoint page hosting the iframe with the Wisej.NET app. The WIsej.NET app need to access and interact the SharePoint page DOM and the JavaScript object containing the Sharepoint context. So my question is: considering that the SharePoint Online run on sharepoint.com and the our Wisej.NET application in another domain how we can allow the Wisej.NET application running  into the iframe interact with the hosting SharePoint page?