Current in the JIRA Jelly tags, the jira:AddCustomFieldValue tag is a subtag of jira:CreateIssue.
What would very useful (especially for people writing custom migration scripts!) is have a similar tag that is "stand-alone". In this way the project id + custom field id or name could be specified with it's value and the following one of two things could be performed:
1. On a single-value custom field, such as any text field or select-list, the value is set.
2. On a multi-select or multiple version picker type of field, values are set and/or appended. (that way multiple values could be set within conditional statements and loops... )