I had a bit of a go at this as an experiment. There is a JSFX in the stash here -
https://stash.reaper.fm/v/38471/Note...dRepeater.jsfx
This JSFX repeats a note at the note's frequency. Adjustable tuning. Repeat frequency can be modulated with pitchbend or CC messages. Will continue repeating while the note is held down. Sends a lot of data and can get overloaded, so maybe not very useful.
It doesn't have adjustable note length as requested by the OP. 400% is not possible in any case, because a note-off has to be sent before each new note-on, so in this JSFX the note length is set at 100%.