[mpl]

If I understood your question right, you can do custom action:

• add stretch marker at position on item under mouse cursor
• take a look at this script, note it uses SWS/Breeeder API

Result - after placing mouse pointer at some position on item and running this custom action by shortcut firstly the new stretch marker added to a given position (say, kick drum you want to stretch), then this stretch marker moved to closest grid

Markers with negative cofficient will damage you project- Reaper will crash if you willn`t avoid that, or you`ll should find and clear some lines of your RPP ). Here is simplified scheme I`ve done it in some of my scripts:

      
-- negative coefficient SM protection
data_safety_distance = say, 0.001 second
for i = 2, SM_count do
  if SM[i].srcpos < SM[i-1].srcpos then
     difference = SM[i-1].srcpos - SM[i].srcpos
     SM[i].srcpos = SM[i].srcpos + (difference + data_safety_distance)/take_rate
     SM[i].pos = SM_t_raw[i].pos + difference + data_safety_distance
  end
end