Yes, you are right. PD4ML does not substitute placeholders for headers/footers defined that way (however it works for inline headers and footers, i.e.
It is quite a trivial task to replace the placeholders in someHTML before it passed to PD4ML. For example, with String.replaceAll() Java API call.
If the workaround, for some reasons, is not acceptable by you, or if you want to keep the placeholder replacement mechanism of PDF generating module in your application consistent, we could add the feature to the forthcomming release.