Looks like the workaround I posted does not work every time... Seems to be rather random. If you keep switching Families and Family members around, it will eventually allow you to set parameters, however "eventually" can be the next switch or a very long time.
Also on opening the .cap again, the Effect parameters will not be visible in the Event View - the Action will simply say "ObjectName:", copying and pasting it will result in the copy having the proper text, like "ObjectName:SetParameterToWhatever".
I'll post under the R2 release thread in Discussion... Perhaps someone involved with coding will chime in - I've looked throught the Effect shader files and the issue seems to lie in Construct.