Don't know if you're still interested in self drive into Myanmar but the info below came from some email correspondence with a Myanmar based tourist agency in May 2012:
1/ "Many thanks for your mail. We have pleasure to hear that you intend to travel with your own vehicle into Myanmar. We can certainly help you with arrangements, however you will have certain restriction to respect which until now are still in place
- Foreign cars need to give a detailed programme to the Authorities of the day by day routing. This then needs to be approved.
- You will have two choices of ‘escort’ or guide: either we put one person into your vehicle, or if this is not possible, then you will have a car driving with you as an escort.
- Accommodation: has to be in government approved hotels or guesthouses, you cannot sleep in your car where ever you like (camper car).
- Border entries: You can enter by car from the Chinese side (Ruili/Muse) and exit at the Indian side (Tamu) provided you have all the necessary authorization to travel through Manipur (India). This can be a tricky part.
- Entering from Thailand: as per my current information, the borders with Thailand are still not open for tourists. You can enter from the north of Thailand into Kengtung area, however you will not be allowed to travel further into the Shan State. Hence, this entry point is of no use to you.
I am planning a meeting with the ministry of tourism sometimes next week, and I can bring this subject up to get hopefully more updated information. I will get back to you no sooner I have had the meeting."
2/ "Thanks for your patience. I have met with the high up authorities, and for the time being, self driving can be arranged for caravan groups only. We would need to apply for permission to travel where we need to give the exact routing and time.
Entry from Thailand is only possible in Tachilek, hence your overland drive into the country does not work, as you cannot do the overland part from Tachilek to Taunggyi. Other borders with Thailand are still not open for tourist vehicles.
The easiest would be to enter through China (Muse/Ruili) and exit the same way again.
We can assist you with the permits provided that you give us an exact routing which we can submit to the authorities.
As the country is opening fast, you might want to wait for a year or two by when the situation might be different and requests like yours would actually be possible without too much hassle."
So, its possible to enter Myanmar on a self drive trip but the restrictions on entry/exit points and on travel in country don't look appealing.