Thats where I wanna hold a cyclocross race...
On the golf course? ;-)
IIRC Dalmeny House already hosts a triathlon, so they may well be amenable to CX (so long as you keep off the greens).
I once did the coastal route through the park on my road bike - unlikely to do that again. However, I frequently go in at the gate at Cramond Brig. The path's rough for 400m or so, then becomes tarmac and S-bends up a low ridge.
You can then take a left to get a good short, sharp climb (I used this to test leg strength/fitness when I started out cycling - you can really see yourself improving as you get up it in a larger gear than before). Then it's a right to head down a fast drop towards Dalmeny House, where you can turn left, up a longish climb (good tarmac, relatively new) to exit on the B924 to either go straight over to Dalmeny or a right to drop under the bridge to S Queensferry (a nice road, but occasionally frequented by boy racer types).