The %m tag is used to define a "margin". To use this tag for the wps you need to be using a patched build of rockbox that has the scrolling margins patch applied to it (which allows the use of the %m tag). While this is a viable option I wouldn't recommend the use of the %m tag on one line of your WPS, you would be cutting off a lot of users from using your WPS, many people like to be up to date with the current build rather than using the (slightly) outdated unsupported builds (granted, they are not that outdated, but any build older than the latest SVN commit is considered old).
Unless you are using an unsupported build to test your WPS, I'd say the %m tag is the cause of your iPod's unresponsiveness.
If you used instead:
%al %pt %ac%pv %ar -%pr
that may work, the spaces will be displayed first, then the time aligned left. Then the volume aligned centre, then the spaces, followed by time remaining aligned right.
(haven't tested this out mind you, so can't say if this will work)