You are correct, the methanol is the cause of the deterioration of the natural rubber. It causes premature drying and cracking of the rubber. As for commercially made Bio-diesel they do not wash it, some may use a product called magnasol ( I am not really familiar with it so cannot comment) However most due to time intensive constraints do not wash their BD they rely on complete conversion.(Blindly trusting all methanol and lye was used in the reaction and settled out)

In answer to your question those who are using natural rubber lines for mods, YES they are at risk, however synthetic, preferably viton is a rather cheap fix.