if i would do something like that, i’d probably just program the pattern and the bounce it to a sample and play that in a loop.
another way would be to program your 6 bar pattern for 12 bars, and make your 8 bar pattern 12 bars (8/2x3), or something like that, at least until you end up with equal length patterns.
i do think that what you want might (in the unforseeable future) be possible using clips, which are (sideways?) discussed here: https://forum.renoise.com/t/concept-zoomable-pattern-editor/21429, and in other places as well but got tired of searching. so you might want to join in that discussion, if your point has not already been made over there or somewhere else, in which case you might want to just add your +1 
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