I am having a gathering at my home on Sunday. I have a WB(utter)SC cake in the oven now. I have approximately 2 1/2 cups of egg yolks in the freezer that I accumulated when making this white cake and IMBC. Can anyone recommend (and provide a recipe for) a filling that tastes great and will enable me to use these yolks?

Gosh, wish I could help. I have the same problem. Love making the WASC. Anybody???

First thought that comes to my mind is to make a custard or pastry cream for filling cakes or a creme anglaise sauce to go with a cake or even a batch of homemade icecream.

Here is a recipe for a custard:
http://www.epicurious.com/recipes/recipe_views/views/104435

and here is one for creme anglaise:
http://www.epicurious.com/recipes/recipe_views/views/4984

and one for pastry cream:
http://allrecipes.com/Recipe/White-Chocolate-Pastry-Cream/Detail.aspx

I haven't made anyof these recipes, but they had good reviews.

http://www.epicurious.com and http://www.allrecipes.com both had a good selection of custard recipes and recipes for flans and custard pies, too.

My Grandma's Coconut Pecan frosting uses egg yolks. I've never tried it on anything but chocolate cake, but it might be good. Let's see...her handwriting was atrocious! Here we go:

Coconut Pecan Frosting

1 cup canned milk (evaporated)
1 cup sugar
3 egg yolks
¼ lb margarine (or butter)
1 tsp vanilla

Combine above ingredients in a sauce pan, cook over medium heat 12 minutes, until thick. Add about 1 can Bakers coconut & 1 cup pecans. Beat till cool. (I just let it set till its cool.)

I hope you can use it for something!

I'd make homemade ice cream! I made some this week to use up some left over cream, and MAN was it yummy! Gelato is another good choice to use upto 9 yolks at a time. There are lots of recipes out there, but I find that the ones from Cuisinart are really no-fail.
Here are a couple of their ice cream recipes:
http://www.cuisinart.com/recipes/recipe.php?recipe_id=163
http://www.cuisinart.com/recipes/recipe.php?recipe_id=103
and gelato recipes too:
http://www.cuisinart.com/recipes/recipe.php?recipe_id=108
http://www.cuisinart.com/recipes/recipe.php?recipe_id=109

I know you are requesting for a filling suggestion but when I read your post, creme brulee immediately came to mind. Yummy!

How to Use Up Extra Egg Yolks:

http://forum.cakecentral.com/cake-decorating-ftopict-24043-.html

http://forum.cakecentral.com/cake-decorating-ftopict-51021-.html

http://forum.cakecentral.com/cake-decorating-ftopict-18632-.html

http://forum.cakecentral.com/cake-decorating-ftopict-4191-.html

http://forum.cakecentral.com/cake-decorating-ftopict-1704-.html

http://forum.cakecentral.com/cake-decorating-ftopict-49276-.html

Egg Yolk Recipes:

http://forum.cakecentral.com/cake-decorating-ftopict-110191-.html

http://forum.cakecentral.com/cake-decorating-ftopict-45491-.html

http://forum.cakecentral.com/cake-decorating-ftopict-35716-.html

http://forum.cakecentral.com/cake-decorating-ftopict-28125-.html

http://forum.cakecentral.com/cake-decorating-ftopict-24845-.html

Sugar Cookie Recipes:
(Uses egg yolks.)

http://forum.cakecentral.com/cake-decorating-ftopict-12292-.html

http://www.recipezaar.com/63803

Chocolate Egg Yolk Pudding:

http://www.astray.com/recipes/?show=Chocolate%20egg%20yolk%20pudding

Lemon Custard Filling:

http://allrecipes.com/recipe/lemon-custard-filling/detail.aspx

White Cake w/Lemon Pudding Filling:

http://www.cdkitchen.com/recipes/recs/29/Mirror-Cake79058.shtml

Egg Yolk Sponge Cake:

http://allrecipes.com/Recipe/Egg-Yolk-Sponge-Cake/Detail.aspx

Assorted Egg Yolk Recipes:

http://www.gourmetsleuth.com/leftovereggyolks.htm
(Also has info on substituting different egg sizes in recipes.)

HTH