If you're not doing a deep bore, I use an endmill slightly smaller than the bore size offset in my boring bar holder. Cheap and effective.
I'd love to see pictures of these. I never thought about using a end mill for a boring bar. It sounds like a great idea for short depths.I used a piece of 5/8 drill rod, turned 3/4 length to .500, with a 1/8 shoulder. drilled and reamed it to 3/16, and made a hack saw cut length wise. I put a 2 flute 3/16 end mill in it and put the whole thing in an adjustable boring bar holder for my lathe. I bored a number of holes to .250 with it, only .200 deep, but after all, that's what I wanted to do.