I use Clear-Lac (who made Tandy's Neat Lac). Has a very slight amount of darkening. Anything that goes on layers on the surface stiffens the product. To me, that's not detrimental to coasters. Surface is shiny. As Fred, says above, you could add talc to make it more of a matte finish.
I did some with an airbrush in the hot summer sun. Some of the mist dried before it hit the leather surface. Left a matte finish.
Tom