(This is continuing a dialog begun via email.)
It could be nifty to be able to define a Thunderbird Tag that would be applied to the message in conjunction with a given custom ThunderLink being created. In other words, with each custom ThunderLink settings there could be an additional option to define a Tag to be applied to the message. This can be useful for enabling the user to clearly mark that they've "connected" a given message to whatever other system they're using (eg, they've created a Task for the message).
To further clarify: The idea is that for some custom ThunderLinks I (or a given user) may wish to "mark" the message they just created the ThunderLink for, as way of letting them know that they've already done something with that message.
For example, there are some emails that I want to create a ToDo (or Task) for some future date. In essence, I want to schedule when I will come back to this email and take care of it (eg, respond to it, do something required for it, etc.). However, I would like to somehow mark the message indicating to myself that I've already looked at it and that my ToDo/Task that I have created (outside of Thunderbird, but which contains the ThunderLink I created) will remind me of when I need to come back to this message. When I have a lot of emails I can't always remember which ones I've "scheduled" and then I can end up looking at the same message multiple times uneccessarily.
So the benefit of this feature is very much dependent on the user being consistent. It won't do much good if they Tag a message when creating a ThunderLink, but don't follow through with Pasting the ThunderLink wherever they intended to. But as long as one follows through I think it can serve as a visual indicator to them that they have "linked" (so to speak) that message with someother program.
In my case I might actually create multiple custom links that are identical except that one TL Tags the message and one does not. That way I would only choose to create the TL that will Tag the message (on TL creation) when I want that visual indicator (eg, I'm creating a Task for scheduling when to come back to the message), and for instances where I just want to be able to get back to the message at some point for reference, but am not scheduling it per se, I don't need to Tag it (but I might need the same custom link string if I'm using it in the same software).