You may use $MATCH to do what you want.
The following example should do it for all messages having a subject.
Messages without a subject will not be forwarded, however.
Hope you can solve that for yourself, if it's a problem :-)
:fc
* ^TOSpookInfo(_at_)uiuc(_dot_)edu
* !^X-Loop: SpookInfo(_at_)uiuc(_dot_)edu
* ^Subject:\/.*
| formail -I "X-Loop: SpookInfo(_at_)uiuc(_dot_)edu" \
-I "Subject: SpookInfo: $MATCH"
:Awi
! `cat $HOME/.spookinfo.list`
: :fc
: * ^TOSpookInfo(_at_)uiuc(_dot_)edu
: * !^X-Loop: SpookInfo(_at_)uiuc(_dot_)edu
: | formail -I "X-Loop: SpookInfo(_at_)uiuc(_dot_)edu"
: :Awi
: ! `cat $HOME/.spookinfo.list`
--
:-- Hans Georg --- student of 'Life, the Universe and Everything'
<georg(_at_)ii(_dot_)uib(_dot_)no> (stud.oecon. stud.scient.)
http://www.ii.uib.no/~georg/
,Studenter, ta fri, det er vår !'