trigger run of archive_command?

Hi,

I want to do online backup saving the WAL files. So I set the database into
backup mode, make a backup, and then set the datbase to normal mode with
pg_stop_backup().
To make the backup complete, I need to backup the actual WAL file too, but
this one is still open when I run pg_stop_backup(), so I want to trigger the
archive_command to be sure that I don't miss anything.
In postgresql.conf I have defined an archive_command in postgresql.conf to
tell postgres what to do when the WAL file reaches a maximum filesize.

I use postgresql 8.1.4, so I only can make the archive_command depend on the
size of the WAL files, right now the maximum size of a WAL file is defined
to 16mb.
As a workaround I could create a dummy table, insert 16MB to make sure the
WAL files get rotated, and then I can backup the last WAL file.

But my hope is that there is an easier way, e.g. do I can use a trigger to
force the archive_command or any other mechanism?

kind regards
Sebastian