Provide and use an intermediary parameter during the execution process

There is an action Token: set value which can be used to set a token to any value, either scalar or even complex objects or lists/arrays, etc.

In step 2 or your use case, you would actually use that to set a token with a name you chose to any convenient value and from that on you can use that token for e.g. math tamper, or any other subsequent condition/action.

Hope, this is what you meant. I'm a bit uncertain due to the phrase "through a temporary form?" as there isn't a form for interaction during execution of an ECA model. But during setup of the model, the aforementioned action to set a token value is hopefully what you need. If not, let us know, please.

Thank you for bearing with me. Not quite there yet. I describe a simple use case.:

1. Entity has a numeric field, say field_date.
2. Via ECA I would like to interact with that field as a user during the execution process. One example: rewrite the value of field_date by adding to it a value that I will provide. Say add +30 days to all dates that I selected (via ECA VBO, and ECA Tamper).

The point is that the value which should be added to the existing field_date field value is provided on the fly, during the execution, for instance through a modal dialog form. (There is such an option to interact within the Flag module (Form behavior - Modal dialog)).

So, you need a form in the first place to provide "the value". You can either use the eca_form sub-module to build a form and then react upon form submission. Or you write an eca_vbo event, which would then allow you to provide some value upon execution.

Not sure, if a generic eca_vbo event to provide an arbitrary value - i.e. without knowing anything about the value's purpose - might be valuable. That could be a possible feature request for the ECA VBO → module.