I've determined that for one of my experimental conditions, the simplest way to integrate it into my current use of <batch> elements is to create a script that has as little content as possible, and really serves almost exclusively to redirect participants to my survey experiment.

How should I go about doing this? What are the minimum components required for my mock experiment script?

A minimal script consists of a <trial> and a <block> running the trial.

Explain exactly what you want to do, i.e. how you determine who is supposed to be directed to either your survey or the standard finish page, and I'll show you how.

What I'm planning on here is several batch elements, one of which directs participants (at random) to this "mock task"/minimal script condition. All participants who are randomly assigned to this script should then be directed to the survey experiment. 
Does that make sense?

It does. You should handle that in the respective <batch> element directly then, per /onscriptend. I.e. something like this (that last condition is the mock condition):

<batch 1>
/ subjects = (1 of 3)
/ groupassignment = random
/ file = "task1.iqx"
</batch>

<batch 2>
/ subjects = (2 of 3)
/ groupassignment = random
/ file = "task2.iqx"
</batch>

<batch 3>
/ subjects = (3 of 3)
/ groupassignment = random
/ file = "mocktask.iqx"
/ onscriptend = [if (batch.3.currentscript == "mocktask.iqx") {
defaults.finishpage = concat("https://yourwebsitehere.com/?subjectid=", script.subjectid);
};
]
</batch>

I've put the above up at https://mili2nd.co/elob so you can try this out for yourself.

Thank you so much! What would a bare minimum script look like (i.e. a single page with text informing participants they're being forwarded to a survey)?