Edit: scratch the below… Forgot that \t doesn’t work in Top Bar. Tolriq said they have no plans to add it in either, at least as of a few months ago when I asked about it for this exact purpose.
But for any String outside the Top Bar, if you are not using Space Between/Evenly for horizontal alignment, you can use the \t operator to do this.
So it could look like:
‣I\t%next.track%
And then only the long part will have Marquee. (Note that Marquee doesn’t work too well if you have multiple long parts–only the first long portion will show iirc.)
(It’ll still work with those two alignments, but the spacing will probably be undesirable.)