Ah, I see. I assume you are using an old style mast and an existing sheave at the rear of the mast? You can still use the same sort of arrangement. Go from the exit sheave up to a block below deck and then back down to a cheek block, etc.
If you try to go straight aft from the exit sheave the line will be too high above the CB case and you will trip over it in a gibe or tack. I have not seen anyone use an eye or flip block near the mast foot but I see no reason why it should not work. It would still lead the control line over the top of the CB case though. I prefer the control lines below the top of the CB case and keep the top clear.
Be advised that old style masts are waterproof, the halyards run through the sail groove and the Genoa sheave is in a welded box. That is why people used an external block for the spinnaker with those masts. They did not want to loose their mast buoyancy. The Genoa sheave box may create a problem if you want to lead the spi-halyard through the front compartment of the mast and the old sheaves near the foot do not access the front compartment, only the sail groove.
These old style buoyant masts are no longer made and quite popular with cruisers. Perhaps you can sell it at a good price and get a new modern racing mast with all sheaves installed and a five sheave mast foot? On the other hand, these old style forged and welded masts are almost indestructible and have much better bend characteristics. The speed gain of having the lines inside the mast is almost non existent. It is mainly a cosmetic improvement. I have done it but I wouldn’t do it again. It is not worth the trouble and not worth loosing the self buoyancy of the mast.