Critter,
I just got done doing some unscientific (eyeball) calculating for you. You would need to deepen the pocket at the back of the plug approxamately .050" - .100" to make room for the depth of the nipple, then drill and tap the flash hole out (just barely enough meat) for a standard T/C sidelock nipple. The Knight/Uncle Mikes nipples are too long, I checked. The only real problem I can see is the firing pin is rounded for a 209, it is not like the old H&R Huntsman from the 70's with the big flat pin for the #11's. Not real sure how reliable the hammer strikes would be, but my gut tells me it would work. You would also have to make sure you bottom out the cap, so it don't strike the standing breech when closing the action. If you have a lathe, you could whip one out in a matter of minutes. Another thing to consider is the powder channel through the plug is approxamately 1/4" diameter and about twice the diameter of my Knight #11 plug, but my Whites #11 plugs have a larger but very short powder channel so it may not even be an issue. I would be interested in converting one of these to #11 too. If you have the tools (lathe, calipers, drill & tap), I have a spare plug and everything we would need to see if it would work before you buy one. I drive right through your neck of the woods 3 nights a week for school, let me know.
The company I worked for has been in business in the same building since 1873, turning a double digit profit every year, just decieded it is closing down the plant and moving our jobs to other plants before the end of the year. So, I am going to school full time right now. Never thought I'd be going to college with my kids, but here I am. It sucks changing careers at 45, but the Auto Industry is leaving the state so fast, it leaves me no choice. Besides moving out of state that is.