Mike:

You want flaps? Do it.

Split the ailerons as you had thought, just be sure to mix them in as flaperons, then you lose nothing except the added weight of two servos and the linkage.

Yes, you could extend the upper surface to make split (scale) flaps, and it probably wouldn't make any difference in the way the plane flies, but it is NOT a scale plane, and it's a lot more trouble.

Bill.