I totally missed this comment and I knew you had added one yesterday. Came back and poof - my eyes saw it this time... Sorry about that.
Another aspect we are introducing into our SES CDP program for the first time is 'executive coaching'. I'm anxious to see how it goes, if you are set on a 52 week rotation that might be a worthy item to add in if you have the funds or can use a certified coach from another agency.
The backfill - I advocate using a version of the pay-it-forward concept. An opportunity for someone begets an opportunity for someone else lower in the organization. If you have several layers of development programs you can hopefully, as part of succession planning, identify high potentials lower down in the organization who would benefit greatly by back filling.
Takes organization, time, and further extends the mentoring needed but it pays off in the long term from my perspective.