I know what you are describing. I will need to give this some thought. I might have a solution in mind, but I want to try it out before suggesting it.
Let me get back to you. :)
So! I'm not sure if this is going to work for you or not, but I have found one possible solution. It involves firing 2 tweens at the same time and effectively switching between them by lerping to the value of the desired tween. It's a bit funky, but it might do what you need it to. Do note though, that both tweens should use the same duration for this trick to work best. Here is a link to an example project. The code is in obj_Example. When running the example, click the left mouse button to toggle between the 2 tweens.
Let me know if this helps at all.