But if it's RCD protected, don't you just need to meet ZsIn = 50V? (i.e.
1667ohm for 30mA)

Technically. Although they also specify a 200 ohm maximum, even this is a
recommendation, I believe.

To me, the regs are clear that it's *preferable* to design for TN systems
to
disconnect on overcurrent rather than RCD, but seem to allow the use of
RCD's instead. (413-02-04 (ii) and 413-02-16).

What am I overlooking?

It comes under bad practice. You could certainly attempt to argue the toss,
though, if it came to court or something. As I suggested, just adding an RCD
to the existing system I would regard as sufficiently safe, although much
over 3 ohms and you're starting to look at lighting and fixed circuits, too.
However, I would not expect to see such shoddiness in a new or rewired
installation and would regard such an installation as a cowboy bodge.

Best practice is to always design main equipotential bonding and circuit
cable sizing/resistance for TN-C-S, even on a TT or TN-S system. That way,
the system can just be transferred to TN-C-S when it becomes available,
which is increasingly common, even on overhead lines.

Christian.