First, a simple answer. No. A Miyata seat is not compulsory.
I have done 20+ miles on a 20 inch uni, and 30+ miles on a Coker, and regular 10 - 15 mile rides on and off road on 24, 26 and 28 inch unis. All but the 28 have Viscount seats, and the 28 has a Velo seat. Both seats are comfortable for the first 30 - 40 minutes, or possibly an hour, then they start to become intrusive. Then I break the ride up with rests, food and drink stops and so on. How far do you plan to ride? How long in the saddle?
Don’t forget that on a MUni, you spend less time sitting down - especially if you are doing the sort of MUni where you need a handle.
I find that the Viscount is a better seat for MUni than the Velo, which is why a moved the Velo to the 28. The Viscount is that bit longer, and is easier to grab and pull when extra torque is needed, usually on steep descents. On uphills, I find that balance and timing do more good than brute force. I have ridden unis with handles, and have never been convinced that the extra ‘leverage’ outweighs the loss of body movement. Others may disagree.
As a general principle, I’d say if you don’t know what you need, don’t worry too much. Almost any good quality seat will do. Same goes for frame, wheel size, tyre, cranks and pedals. Until you’ve used the set up yourself in ‘real world’ riding, you won’t know what suits you. What sort of trails and tracks are available to you? Level but muddy? Steep but well made? Completely unmade? Will you be on an artificial BMX/MTB course? Or do you enjoy jumping and hopping? Are big drops your thing, or do you think there’s a reason why wheels are circular - and it’s that it makes them roll better?
I’m not being negative here. In fact, I’m being very positive: unicycling is fun, but it doen’t suit everybody; MUni is great fun, but it doesn’t suit every unicyclist. With the right attitude and determination, you can ride ‘off road’ or ‘cross country’ on anything from a fat-tyred 20 to a Coker; on a big-cranked 26, or a 24 with microcranks; even on an ultimate wheel - with no seat at all! Choosing the saddle is not a major stumbling block. Enjoy your MUni, and you will hardly notice the saddle.