[O] set minor mode based on first line of a buffer?

[Matt Price]

Last question for a while I hope!

I have a whole bunch of org files that I am reposting to my website at
the beginning of the school year.  These are files generated with
org2blog/wp, and htey all have afirst line like:
#+POSTID: 1797

I would really like to tell org-mode that files that start this way
should be started with org2blog-mode (a minor mode) switched on? I
guess I need to write a hook that parses reads that first line
somehow?

thanks again,
matt