Think what you really want is to control the [i]Selection[/b] then, rather than slice markers, otherwise you are still going to run into exactly the sample problems! See my initial reply below.
MIDI Control selection start/end. (See below on previewing.)
Shift+Alt+Up/Down (or MIDI map again if possible, it most likely is on the API) to select sample slot.)
(Copy and Paste could probably be assigned to MIDI buttons too.)
Using slices is never going to work for this! Maybe a slight problem with previewing selection over MIDI? The Return key does it from the computer keyboard though and it’s likely to be in the API. Seems to want a fair number of button in total but that is fine as long as it’s a single mode/page of the controller I guess.
I think this should really be a Tool/Script suggestion, rather than a general Renoise one…
Original reply, still thinking in terms of Slices.
What you’re suggesting isn’t going to change that. Sliced samples are always mapped across the range and not free like a normal instrument!
Sure you might like to be able to adjust start and end point by MIDI, a little tool that will sense where the cursor is and then adjust the markers either side of it with a couple of knobs. How would they work? A middle area = no change, lower moves marker back (getting faster), higher moves forwards (getting faster) as with MIDI’s 127 values you would have less precision than the 09xx commands if it was done discretely.
But still you are going to have to copy your slices into a new instrument.
I would suggest getting used to shortcuts.
Slice you samples in different instrument.
Set Auto-Select Played to On.
Play the keys to find slick you want.
Alt+Down to Instrument
Shift+Alt+Down to Sample slot within Instrument.
Ctrl+V to past new sample.
Generate Drumkit when done (or manually map, use one of the available Tools.)