Kevin C: many thanks! The final item in your first link pointed me to the following solution, using another command from pdfcomment.sty:
%% popup acronym expansion
\gdef\acp#1{\pdfmarkupcomment[markup=Underline,color={1 1 1},author={{#1}}]{{#1}}{{\acl{#1}}}}
which together with \newacro{acronym}{full expansion} of the
acronym-package indeed does the job, when the PDF is viewed in acroread and Adobe reader for Mac OS X. It does not open the popup in evince, okular, or Preview (Mac OS X built-in tool), but at least it does not generate the ugly black boxes in Preview, that I had from \pdftooltip. So I can stick these popups now in any PDF I produce and if my readers are smart enough to use Acrobat, they will get the popup explanation at any acronym they hover over. Should be standard in scientific papers! If they use another viewer, at least there is no fatal interference.
I hesitated about underlining such hoverable acronyms in blue, but chose not to mark the pop-ability at all. Let the reader discover by herself. Hence defining the white color.
Apologies for not knowing how to typeset the LaTeX code above. Typing in this site is fighting my emacs habits.