Turboprop pilots add a fuel icing inhibitor called
Prist. Very toxic, stinking and tedious to apply. Some fuel stations serve kerosene with Prist already mixed in, others don't. In 2009, a
PC-12 crashed in Montana because the pilot did not apply the mandatory icing inhibitor.
So it is possible to manually apply mandatory fuel "enhancers". Lycoming/Continental could just issue a SB allowing the use of 91UL + octane enhancer. They have done that in the past, e.g. when 100LL replaced the higher leaded 100.
I'm sure this octane enhancer would be a nice substance, very useful to put in your neighbor's fish pond when you're having a little quarrel.