I picked up a rearend with coarse spine axle. If I take along the coarse side gears, could they be used for spares in a 78 fine spline rearend?

I have changed coarse axles out for fine spline ones with the side gears, so I dont see why you couldnt go the other way, for a trail spare.

So long as you carry the inner side gears, the axles will slip in.

However, that means you are likely running an open rear diff, so your odds of breaking rear shafts with open diffs are like .0000000001% or so. Hardly worth the weight to carry them, IMO.