Okay, got this figured out. If anyone else is curious, what I did was edit the selectAppropriateDefaultPlan() function in app.js. I replaced that logic with something like this:
if (this.query.plan) {
this.selectPlanByName(this.query.plan);
if(this.query.plan === "Monthly Plan Name" ){
this.showMonthlyPlans();
}else if(this.query.plan === "Yearly Plan Name" ){
this.showYearlyPlans();
}
}else{
this.selectPlanByName("Default Plan Name");
this.showYearlyPlans();
}