I answered my own question while writing this post. But in true Mastermind
final question style, "I've started so I'll finish..."

I want to have 3 package collections on the same server (dev, test, live).
Each collection has a single shared configuration file. Systems policy is
that test and live packages can't be edited.

However I couldn't see how this could be easily achieved because the direct
configuration file path is absolute, and indirect configuration would have
all package collections using the same path. Moreover, there didn't appear to
be any way to set the file path dynamically using variables, property
expressions etc.

I tried setting up project data sources because the .ds file is easy enough
to edit, but unfortunately it doesn't get deployed.

I didn't want to manually edit the connectionstring attribute in the .dtsx
files on test and live because this would make a mockery of the policy. But
this led me to thinking that I could edit them in dev and put a relative path
in - kludgy but the best solution if it didn't kill the packages. And then
while checking that the packages survived the experience, I suddenly realised
that in the Package Configuration Wizard, you can just type the file name
into Configuration File Name instead of using the full path generated by the
Browse button! And you can also use a relative path!

Shockingly it took me 2 days to work this out. I hope this spares someone
else 2 days of keyboard imprints on their foreheads

David