We are trying to filter the chat interactions in the Interactions page using a custom attribute (unique to each client). We did come around "Set External tag" because that's already been under usage in Inbound call flows. According to this documentation, Set External Tag action - Genesys Cloud Resource Center seems like cannot set external tag for web chat flows. We would like to make use of the concept of querying our chat interactions with a custom attribute.
Need suggestions on any other alternatives for this ask!
Unfortunately, I don't think there is anything available apart from External Tag to achieve what you are looking for.
I just checked and it seems the limitation on webchat interactions is only related to the Architect flow and the fact that there is no "Set External Tag" action available in Architect Inbound Chat flows.
So you should be able to set the external tag via Platform API - using PUT /api/v2/conversations/{conversationId}/tags
You can create a Genesys Cloud Data Action that leverages this API endpoint and invoke it from your Architect Inbound Chat flow.
I must say I haven't tried (via a Data Action) but I think the API endpoint can be invoked with Client Credentials token (what Genesys Cloud Data Action uses).