Update #2 Success (step 1?)
I installed my brooder wall at the front of the drop boards yesterday (1st day with manageable temperatures). I left one of the large opening panels off and put another board at 90 degrees midway of the drop board length (closed off the side containing the favoured back corner).
This morning I found six eggs in the new roost box, expect another one around mid-day hoping it too will be in the nest box. Front the left side I had one egg in box 1, four in box 2 and one in box 3. As @jthornton suggested closing off and forcing the move had the desired effect.
[PM Update: My 7th egg came around 2pm laid in nest box 2.]
My thought is to leave the brooder wall in place for 2 weeks and then remove, guessing that this length should be enough to burn the new behaviour into their little brains? Anyone have any different suggestions?
PS While I was re-installing the wall I thought about how we had to train our initial ISAs to use their roosts but our hatched Ameraucana mixes were imprinted by older pullets. Expecting that the 4 Ameraucana pullets will be imprinted by the ISAs to use the new nest box..