Depends on whether it is contested. Most registries issue the subpoena, and it only comes to the Magistrate’s attention when it’s (a) contested or (b) ignored and needs to be enforced. Despite that, they do have the power to vary the terms and scope of the subpoena. It is an order from the court itself that the applicant requests they make, and just like any other order it is up to the court as to the form which they will grant it in.
To appeal, you need a decision to appeal against. Here it appears you’ll be appealing against a decision to adjourn until next year, at the end of the year when all the court calendars are likely full (and it’s probably doubtful you’ll even get in to the appellate court before the adjournment is up), when there is usually some form of reason why the adjournment has been made (such as to allow the other party time to make submissions, or the court simply doesn’t have time to deal with it at the moment, or it should be raised by way of application and notice given to other interested parties but hasn’t....). As long as the adjournment is in any way reasonable, and especially if it has anything to do with ensuring due process and sufficient notice to all interested parties, your chances of success are probably going to rest right around 0%.
I think ‘trying to appeal it’ is a good way to annoy two courts - the one you’re appealing from by trying to go around them, and the one you’re appealing to for wasting their time.