How about a thin length of string soaked in paint, held taught . Wouldn't want too much paint on the string so you wouldn't goop up the holes. Just slide it down into the slit and move it back and forth a bit.And how do you want to do this? Yeah, I know that you an asking but I really do not see an elegant way to do it.
I BTW wouldn't try this. But let me know if it works (ha, ha).