Picture effect

[Ebuechner]

Before I put a official enhancement request on GitHub I want to check to see if this is even possible.
I was looking to see if the frame rate adjustment in the picture effect could be set so you could run a negative number to run a gif backwards and possibly have a value curve.
Plus maybe a way that you could hold a certain frame from a gif image.
For instance run a gif image up to the 7th frame hold for a few seconds and then continue on from there.
I'm pretty sure something like that may have to be done with more than one picture effect run back to back.

[Ebuechner]

So no input on this?
I'm asking the question because I'm not sure if animated gifs would allow any of this.
I don't see a value curve on the frame rate adjustment which means that this might not be something that's feasible that's why I'm asking the question.
On the second part of it I was picturing a checkbox with a slider next to it with dual controls so you could set a start and end percentage or frame.
Unless there's already a way to achieve this that I'm missing?
Or this questions already been asked and answered.