I broke the code
Dane: Much easier then I thought. I have got 2.76" round pucks. The bulbs are the size of the end of your pinky nail. t-3/g-4 (3 = diameter of the bulb, 4 = distance between the 2 plug in prongs). The transformer (buried in the ceiling: my German father-in law did most of them) might work or not.
The LEDs will plug into the halogen fixture (mine plug in horizontally). Each halogen runs on 20 watts, thus 6 run on the 120v transformer. The LEDs will pull a very small load. If the transformer type needs a load (some don't) then the lights will flicker and cycle on and off. If this occurs, use one halogen for a load and 5 LEDs. If it doesn't flicker, all LEDs.
In either case, I won't have to tear up the ceilings
I ordered 7 as a test run from superbrightleds.com out of St. Louis.
I ordered "warm white" which will mimic the halogen.
I would look at replacing them soon: life of halogen is 2,000 hrs and LED is 50,000.
I will let you know how it goes.