Dave,
You didn't mention what year your boat is. That might be helpful info.
I have a 1975 F26. It does have a hollow keel, which was not originally accessible. I was originally alarmed when I figured out that water had seeped into this hollow space through screw holes used to mount various items, such as the forward bilge pump on what I call a "false floor" that caps off the "sealed" compartment which forms the hollow keel. When I originally opened this space it was filled with nasty smelly sludge. I call the floor "false", because it is a relatively thin laminate (about 3/16") and is separate and distinct from the outer hull.
The hollow space inside the keel is just over an inch wide at the bottom, and about 4 inches wide at the top. At the aft end of the keel, the height of the hollow space inside the keel is about 6 inches. The height tapers down with the height of the keel going from the aft end of the keel forward. Stagnant water trapped in this hollow space creates odor problems and freezing in this enclosed space can be damaging.
Below are two photos which show the hollow keel. These are from a project I did last season, when I added a 6" Mahogany keel extension to improve tracking stabilty. The first photo was taken while fiberglassing the keel extension to the hull. The original keel line is vissible in the first photo. A hole in the keel which I drilled to install a through hull fitting is also vissible in the first photo. The plug drains standing water from the bottom of the hollow keel during the off-season preventing accumulation and potential freezing of water in the hollow keel.
The second photo is I think what you are asking about. I included the first photo as a point of reference. The second photo looks more or less straight down at the "false floor" where the drain hole was drilled (after the second photo was taken.) In the second photo, fwd is to the right, port is on top, starboard is at the bottom.
In order to install and access the drain plug, I cut this rectangular hole about 3-1/2 inches wide and 7 inches front to back providing access into the hollow keel compartment. Since the compartment is no longer sealed, I installed a very small pump with a float switch inside this space to keep the keel compartment as dry as is practical. Since the compartment is only a little over an inch wide at the bottom, the pump won't fit all the way to the bottom, so there is usually a little water left in this area.
To be safe, you should avoid removing plugs below the waterline while the boat is in the water, unless you are absolutly sure you are not going to flood the hull. [If you arent' sure from looking inside the hull if the fitting goes through the hull, dive down and examine the outside hull to make sure removing the plug isn't going to flood the hull.]
Good Luck
& Merry Christmas!
ChuckA