Is there a way for Excel to structure a mortgage style calculation so that
yearly payments are equal but where principal and interest are not paid at
the same time? For example, interest would pay monthly but pricipal would be
paid on a yearly basis. (actual payment dates would be the same, meaning
that interest would always pay on the first of the month and at the end of
the year, principal would also pay on the first of the month). In other
words:

Jan1 Interest
Feb1 Interest
....
Nov1 Interest
Dec1 Principal and interest

The PMT function is set up so that principal and interest are always paid
together.