I've not had good luck using lock miter bits on any kind of plywood. I've just had too much tear-out of the center wings of the joint, and yes, I have the Lock Miter Master Set up guides. I would use one of the other locking miter joints that have been suggested using the table saw to create them.
For cross grained splines, I use a table saw tenoning jig, and set it so that the spline is the board face and between the jig face and the blade. Doing it this way allows me to make two splines by just flipping the board over to cut the second, and then I can repeat the process on the opposite end of the board for two more splines. I then use my miter saw set to 90 deg with a stop to cut the splines to length (length being in the direction of the grain). If I need more than 4 splines, I just repeat the process to get 4 more splines, etc. For a 24" miter, multiple splines can be used end to end to achieve the 24" I've made many splined miter joints this way, never had a failure, and it's entirely done on the table saw.
Biscuits are another quick way to spline a miter joint easily, but build yourself a jig for the biscuit joiner to get accurate cuts for the biscuit slots.
Charley