If this is not possible, what is the best process for ensuring that users read and agree to both ESRI's and our organisation's terms and conditions? Possibilities I have considered include:
If using one of the above workarounds, what link(s) can we use to direct users to the ESRI terms and conditions for ArcGIS Online?
That still leaves the Esri Terms and Conditions. You bring up a good point about community user you are inviting/adding behind the scenes. Some Hub customers use Survey123 to collect and vet community users. You might consider delivering terms and conditions as part of the survey: https://community.esri.com/t5/arcgis-survey123-questions/adding-accept-terms-and-conditions-to-a-sur...
Thank you for your feedback and questions.