Andy F. , I'd use the PUT /api/v2/tickets/{ticket_id}/tags endpoint instead of PUT /api/v2/tickets/update_many, as the latter creates a background job, which may be rate limited to 30 concurrent jobs for the entire instance.

You could use the Add Tags endpoint, as mentioned above, with a payload such as the following:

{
{%- assign randomizer = ticket.id | modulo: 2 -%}
{%- case randomizer -%}
	{%- when 0 -%}
		{%- assign randomTag = 'control' -%}
	{%- when 1 -%}
		{%- assign randomTag = 'experiment' -%}
{%- endcase -%}
	"tags": ["{{ randomTag }}"]
}

As for your other question:

How would I use liquid markup in this JSON response to tell it not to fire if a ticket already has the control or experiment tag?

The following trigger conditions could help you exclude this from firing on follow-up tickets with the tags:

Ticket Is Created
Channel Is not Closed ticket
Tags Contains non of the following: [ control, experiment ]

Of course, it'll depend on your specific use case. You could also try iterating over the {{ticket.tags}} placeholder and use Liquid's contains.