I would like to do that. I will have to check my schedule but that sounds like a great idea. 

Technically I think any exterior change has to be approved, realistically you'll find that few people have landscaping approved.  I would say the bigger the project the more you might consider approval, especially hardscaping but landscaping probably isn't a big deal.  I've had a couple of different things approved by the architecture committee and it's really easy, you fill out a form and provide a drawing and they get you an answer in a couple of weeks, I've never had them say no to anything.  All the links are on this website.

I do not believe anything has changed. Tall fescue is all that is allowed at this time.

Hypothetically, if you did put it down, I think nothing will happen unless someone calls PPM and complains.  But since zoysia will naturally spread (slowly) you might want to consider how your neighbors might react.

If you do get "caught" the consequence is that the HOA could demand that you rip it out and put down fescue at your expense.  The HOA could also fine you (after a couple warnings) and if you ignore the fines I think they can put a lien on the house.  Zoysia is pretty expensive so it would cost a lot if you had to rip it out.

Since it does spread you could just slowly mix it in to your lawn and let it overrun the fescue naturally.  Others might not notice as much if its a gradual change although zoysia does stand out in the winter when its brown/dormant. (not to mention in the summer when its green and the fescue is brown/dead). Personally I decided not to risk it but I do loathe fescue and I'd really like to be able to put down zoysia.