I was a SENCo in a small secondary school and included on SLT. I worked in primary initially, but then became an advisory teacher for literacy support and later, behaviour support. That was useful experience for liaising with outside agencies, working with SLTs etc. I would think it would be difficult to get a DH without recent whole school experience, but what about LA posts? It is possible to get into advisory work and then back into school, but the academies might have changed all that now.