After a bit of detective work, I found this worked...
var sortClause = LoanFields.IssueDateTime.SetAggregateFunction(AggregateFunction.Max) | SortOperator.Descending;
sortClause.EmitAliasForExpressionAggregateField = false;
... but this is the opposite of what the docs say:
Sorting on an expression
When a field with an expression is placed in a sort clause, it gives a dilemma: is this a field which is already in the select list, so an alias has to be emitted or is this an expression to use as the Order By clause? By default, LLBLGen Pro Runtime Framework will choose the former, as it's the most common, and will emit the alias set on the field instead of the expression. To sort on an expression however, the property sortclause.EmitAliasForExpressionAggregateField has to be set to true (default: false).
That property is true by default so I suspect the docs are wrong.