dblue did a Native Monophonic Synth in Renoise which used a custom LFO envelope just as danoise says. in fact, check out this post by danoise in that same thread where he shares the XML for a Pentatonic scale envelope.
dblue mentions somewhere that he used a program he wrote to build an XML file out of values so he did not have to calculate everything by hand, but i don’t think he released that program anywhere on the forums. i could be wrong though. he might respond himself to tell you more.
hope this helps you and good luck with your project!