Hello Royce, what you are suggesting is a very (very ) common practice in boat building. In fact, we order our sifted wood flour in 50 lb bags, and you would be amazed at just how much sawdust you need to get to 50 lbs. I am not a huge fan of West System as a brand, but these techniques work with any laminating epoxy. Do not use the fast setting hardener unless you are working in a cold environment and get it out of the cup within the first few minutes or it will start to harden up on you. Don't use instant hardener unless you really know what you are doing and can work very fast. Remember that epoxy is heat activated and when the cup starts to warm, you are done using it. Using wood flour as a filler will lessen the pot life of the epoxy.
Before I go too much in depth here, I have a video up on my web site called "Using Epoxy as a Glue" you can view it at
http://www.sandypointboatworks.com/w.../video-library
The only difference in using epoxy as a glue and using epoxy as a filler is the amount of filler you put into the epoxy and the types of fillers that you use. As you are trying to stay natural, the only filler you need to be concerned with is wood flour as all the others need to be painted.
In general, all wood flour will darken up significantly when added to epoxy. You do have some control over hue of the final color with different types of wood flour, but that is fairly limited.
As for your concern over the "flow" of the epoxy, it is a non issue. There is absolutely no need to mix the filler into a putty consistency, just mix in enough wood flour until it runs off the mixing stick with a runny consistency that you are happy with and you are done. If you mix it to the consistency of a putty, peanut butter or maple syrup it will make no difference in the final color or the fill.
When building a boat, whether we are filling a crack, making a fillet, Gluing a gunwale or easing the edges on an inside corner to put fiberglass on, the only difference in the mixture is the amount of wood flour in the mixture and how much we want it or don't want it as the case may be to run. Be sure and use sifted saw dust and get out all the big chunks. The finer the better, there is a reason why they call it wood "Flour". a fine belt on a stationary sander will do the trick.
Keep in mind that epoxy sands much harder than wood filler and once you sand it, it will turn milky when sanded. Don't be concerned about this as it will brighten right up with a top coat of varnish or another coat of clear epoxy. Also remember that if you are using this outside, that Epoxy needs a UV protection layer.
When I build furniture, I use epoxy and microballoons as a fairling tool on joints which will be painted for a perfect, shrink proof surface to paint.
Good luck.