I have managed to do multiple-line fontification in Emacs (and I think in
org) using hi-lock mode:
M-x hi-lock-face-buffer <REGEX> RET
italic RET

But I can't for the life of me get that regex back which could span multiple
lines.  I was sure I grouped '.' and the newline character somehow, but I
can't reclaim it.  Maybe someone else has an idea.

If you get it figured out, you can then write the hi-lock regexp's into your
file, and set things up so that they get read every time the file is loaded
(with or without confirmation).

It worked for me in a limited trial but I'm not sure how hi-lock would
interact with org over the long haul.  It certainly has the potential to
slow things down, but I never noticed it.

Scot