Actually I was thinking about computing the new word-space and
baseline-skip based on the one defined on the paper block (ie the
"default" one) multiplied by the magnification. Then, when using
\abs-fontsize is like making a new start, getting rid off previous
font-size, word-space, and baseline-skip. To solve the problem that
was posed (making eg. a title page that looks the same whatever the
chosen staff-size), it may be the simplest and most reliable thing.
I've added that command to git for people interested to play.