I have been experimenting, Quetscher, using masking tape to cover all the holes and then trying different combinations by poking combinations of them open.
If no holes are open, the HM-26 is not playable. If too few are open or in a wrong configuration, some notes choke. The combination shown at the bottom works pretty well, and the Bootay effect can be produced to some extent working just the hole at the far left (lower) end, with the thumb of the left hand. But this is very awkward to do and requires propping the melodica between my knee and my mouth, and the results are not as good as with Lowboy’s technique. The same can be done with the Cassotto, with which it’s easier. I’m still experimenting and also looking at a few other melodicas that emit some of their sound through holes or slots, though they don’t do this as strongly as the Hohners do.
Notice that the two holes on the far right are open and not covered with tape.