For ESD protection, is it wrong to clamp isolated I/O to chassis ground?
No, it's quite common to do this and in almost all cases it's better to have an ESD transient event shunted to the chassis. Chassis grounds can usually handle\shunt large currents better because they are lower impedance and usually closer to the ESD source.
In addition if the ESD transient goes through the PCB it could also cause common mode noise through a low impedance cable and cause noise or worse, so what you have shown will probably work best for your design.
If A and B really are coming into the PCB at 60V because of galvanic levels, it would could exceed the maximum voltage of whatever transceiver you are using (or digital I/O) and cause problems there. If this is the case it may be best to use a digital isolator with built in ESD protection.

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