[Exim] storing filter 'files' in a db

Is it possible to store the text of a filter file in a database? I have
a large mail system sytem in which users do not have shell accounts and
are authenticated through mysql. I'd like to be able to do a forwardfile
director with

file = ${lookup mysql{select filter_file from filters where
login='${local_part}'}{$value}fail}

The documentation implies that this won't work
(http://www.exim.org/exim-html-3.10/doc/html/spec_13.html#IDX958).

Any suggestions?

Thanks,

George