[Exim] bug in transport-filter parsing?

    I was trying to use a expansion to specify the entire content
of a transport-filter specification, but it seems that the trick
of putting double-quotes round the expansion-item (which contains
spaces) to prevent it being split by the parsing into an argv
only operates after the first arg.

   transport_filter =    /some/path "${if = {${lookup pgsql{select blah} {$value}fail}} {1}\
{/path/to/command arg1 arg2} {}}"

   transport_filter =    "${if = {${lookup pgsql{select blah} {$value}fail}} {1}\
{/path/to/command arg1 arg2} {}}"

Has anyone else seen this?    What's a good way to transport-filter depending on
a lookup, but avoid the overhead when it's not needed?

Thanks,
    Jeremy