Solved, and yes it was simple…
oProp.getObject().getModifier(0).getValueChannel().setValue( 1.0 );
Found the answer in Semidieu’s Change shapes value thread, also in the Developer Discussion forum. I’d confused myself because I’d started with a node, not an object (my oProp is a DzNode).
I don’t think I’ll go for the one-line solution I’ve given - a few checks as mentioned in the other thread are probably a wise future-proofing precaution.