I would not worry about oval peg holes as being the primary problem with fit up after air drying. With green boxed heart wood, checking results in displaced and rotated surfaces. To help with fit up, hollow shoulders and abutting surfaces with a slick. I think you could bore the mortised stick but let the tenon alone until fit up and then draw bore. If oval bores are a later concern a few quick swipes with a block plane makes an oval peg but a normally shaped tapered peg may work without problem.
