dear doghouse,
thou there is alot of funny ideas from different specialise dept to
be a standalone dept, but its impossible lar.
as long you do not belongs to part of SPF, the effectiveness of the
warrent card will be affected. TP did once try to merge with LTA
but in the end, i believe due to similiar reason they withdraw the
idea.
to be "no longer under the police force" is never easy, as we are
law enforcer and if you are not under SPF there will be changes to
the statutes of SIngapore. chapters affected will be like Penal
Code, Criminal Procedure Code, Police Force Act etc etc....
so think abt it, will it happen? 
cheers