This is related to and probably LDEV-2181.
Support for certain serialization settings for compatibility with ACF11 has been added to Lucee in LDEV-974, but support for a custom serializer class (using this.customSerializer in application.cfc) has been omitted.
Example use case: The default JSON representation for the current date and time using serializeJSON() with a US locale is "February, 06 2020 11:30:00 +0100". We would like to change that to an ISO-8601-compliant representation like "2020-01-06T11:30:00+0100".
As far as I understand, this is a typical use case for a custom serializer class.