So my question is: how far can we rely on perl to keep SvREADONLY scalars shared?

Not very far, probably. As noted above, type coersion will still modify the underlying data structure. Also, creating references to the data can make the memory unshared if the SvNULL's REFCNT field is on the same page as the actual data. (IIRC the copy-on-write mechanism is on a per-page basis). That's probably especially relevant if you have a lot of nested data (i.e. much of the data is references).

See also http://gisle.aas.no/perl/illguts/

ps: if you keep the parent process alive and periodically kill and refork children, you might be able to keep this under control. it probably depends a lot on the data and the kind of access you need to it.