I have written a custom Token which contains several standard paragraphs on how to make membership payments to our organisation, which will be included in several Message Templates related to email and PDF renewal reminders. This works OK for text and HTML, but gives errors if I try to include a CiviCRM Token within it (e.g. the Contact Id, which we are using as a membership number).
Presumably this is because SMARTY is processing the embedded token while carrying out the insert process, and does not have the necessary context on Contact details or related Tokens. I note that one contributor to question How can I include a custom template file in an email template using smarty? indicates that this may be a problem, but I wonder if there is any way round the problem?