I've made Algernon [1] to explicitly register as a filter.
This solution still overrides a HandMorph method [2].
For now, this is ok for my purposes, we will see if Algernon
maintainers may pick it up.

Thanks for all your help!

Cheers, Balázs

[1] http://www.squeaksource.com/Algernon/Algernon-kb.15.mcz
[2] namely #handleEvent:
It just adds a line after notifiying listeners:

	self sendFilterEvent: evt to: self eventFilters.