We can do this now with the API, when requesting a campaign we can pass overrides to the default tokens in that campaign. We use this to pass fields through for emails that wew don't need stored in the lead record clutering up the screen when reps look at it.
We are converting to embedded forms and I'm jsut trying to figure out if theres a similar way to pass temparay data back with the form without having to create new custom fields.
Well, yes and (I think) no. You can post previously unknown fields with the form and they will be stored in the Filled Out Form activity. But AFAIK they can't be transformed into tokens unless you first used the API to scan the records and then did an API Request Campaign.