Since the epoxy/flour mixture is much harder than the wood alone, it will be harder to plane. It will also dull your plane blade more rapidly. But it should still work.

On my boat I just dripped unthickened epoxy into the predrilled holes. That's not an area that worried me. My spar and boom stay pretty dry.

   Actually when I built my 24 foot tri (before there was a CLC or it might have been a Madness).  I planed all kinds of epoxy, and glass.  The shop was away from where I lived, so only the bare min. tools got there.  I took a japanese plane which has a pretty hard blade in it, and I planed for 6 months all kinds of wood and glass and epoxy and never even sharpened it.  Eventually the blade had to go so far out to catch a shaving it would only do pretty coarse wood, and I finally sharpened it.  I also had to replace 3mm of the sole.  So planing is a possible.  I would be less sanguine about using my jointer because the blades are such a hassle to sharpen.

Just today I was readying a structure for fairing, and I was planing the glass fringes with year old hard epoxy off.  Lot less nastiness than sanding.

Another option is to put guide on the spar if it isn't too thick and trim with a trim bit.

Or make a few wedges to fit under the drill jig.

   For softwood spars, consider not epoxying at all- just varnish the surface and use them. It's true the epoxy slightly hardens the surface, but not enough to help over spruce or fir, and when it gets dented it cracks and becomes orders of magnitude more difficult to refinish.

Like the other posts I wouldn’t worry too much about the drill-fill-drill on the mast unless you plan on spending a lot of time upside down (a consideration I had to make). I found planning epoxy with a sharp plane to be do-able but it was difficult and the finish in the area wasn’t quite as clean. On everything above the waterline I just took a Q-tip and painted the inside of the screw hole.