So how can I pick a random member of that sequence of
files-available-to-be-referenced each time the template matches and I
need to replace the value of @href with that arbitrary actual file
reference?

One way to deal with the "recursive" call requirement of the
random-number-generator function I have experimented with in XSLT 3 is
to use an accumulator with the
initial-value="random-number-generator(current-dateTime())" and an
accumulator rule for the matching nodes to create new one e.g.

So perhaps you can adapt to get a new number based on the accumulator
attached random number generator of a matched node.