You can use an anonymous type instance to act as a template object:
var template = new { PeriodTableId = 0 };
var periodTables = _session.QueryOver<PeriodTable>()
.Where(x => x.ClientPolicy.Id == request.ClientPolicyId)
.SelectList(list => list
.Select(i =>i.Id).WithAlias(()=>template.PeriodTableId)
)
.TransformUsing(new ExpandoResultTransformer())
.List<dynamic>();