Polymark: draw marks horizontally

[caagr98]

I'm using Polymark (LSR 976) to be able to print song names (in 
medleys), codas, etc. at the same time as rehearsal marks. However, 
using a `\polyMark Center` (or empty) at the same time as a `\polyMark 
Left` draws the Left diagonally above the Center (assuming the Left is 
defined last). I'd like to draw them horizontally instead. Basically 
what `\override Score.RehearsalMark.extra-spacing-width` does, but for 
the same column. Is this possible?

To clarify with crappy ascii art, Polymark draws:

 "Song Title"
[A]

I want:

[A] "Song Title"

Obviously, if there isn't a rehearsal mark in the same column, the Left 
should be directly above the barline.