[Exim] lookups for general options

Hi,

sorry if this is a stupid question, but I've been googeling for a while
now, searched the FAQ, the list archive and of course the manual, but
didn't found any answer..

I would like to set up different cascading Exim instances, which should
share some general options like smtp_accept_max, smtp_verify,
timeout_frozen_after, etc...
Instead of hardcoding these values in every configuration file, I would
like to use a lookup (mysql for exemple.. because this system may also
be distributed on many hosts).

I tried a lot of things, but it seems that these options would not
accept any lookup and require a real value.

this will not work:
timeout_frozen_after = ${lookup mysql{select timeout_frozen_after from
mta_config where stage=1}{$value}}
giving me a : invalid time value for timeout_frozen_after

maybe someone has an idea about that ? or am I missing something..

best regards
Olivier