How to add a custom field for TLVs in the SV and PTP protocols?

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The IEC 61850 Sampled Values dissector does not provide a hook for custom TLV dissection. Being an ASN.1 generated dissector, that would require reworking the templating for that dissector.

The IEEE 1855 Precision Time Protocol dissector has neither such hook, so would require changing that code as well.

Unless there's some Lua wizardry that can help here.