I had to be specific because you might get caught in one of the API ‘traps’ such as not being able to reload a slice with a stretched version. Anyway, a more detailed plan always makes coding easier 
To be honest, I haven’t done it because it’s never something I’ve thought of. I’m not very good at ideas usually, but I can do the heavy lifting code-wise. 
When I say in memory I mean in a variable in your script. Presumably a user wouldn’t restart Renoise while it’s time-stretching!
Personally I would store all the data for a single slice (= beat) in a table and then have another table which contains those tables.
For example a ‘slice’ table may look like {32051, 48, 48} where I store original number of sample frames, note mapping low note, note mapping high note.
Then, by placing those in another table you can use the index of the wrapper table for your slice numbers
e.g. slices = {{32051, 48, 48}, {32125, 49, 49}, {31943, 50, 50}}
Your iterators would then be slices[slice_number][param_number]
If I wasn’t so busy on Cells! then I would do it myself.
Hope you have a good move and feel free to ask any questions.