TextSpanner and line thickness

[Mike Solomon]

Hey all,
    I'm working on a piece where the thickness of a text spanner represents
playing intensity, and in writing it, I'm realizing that the thickness of
text spanners has an effect on their X extents (see the minimal example
below).  This is due (I think) to the round-cap line ending.  I would gladly
use non-rounded lines so that they stop consistently at the same horizontal
spot independent of their thickness.  Before I dive into my git repository
to cobble together a solution, if any of you know a way to get around this
problem by standardizing the horizontal lengths of text spanners, I'd
appreciate it!

~Mike

***
\version "2.13.29"

\relative c' {
\override TextSpanner #'style = #'solid-line
\override TextSpanner #'thickness = #10
a b c d\startTextSpan
a b c d
a b c d\stopTextSpan
a b\startTextSpan
a b c d
a b c d\stopTextSpan
}

\relative c' {
\override TextSpanner #'style = #'solid-line
\override TextSpanner #'thickness = #20
a b c d\startTextSpan
a b c d
a b c d\stopTextSpan
a b\startTextSpan
a b c d
a b c d\stopTextSpan
}

\relative c' {
\override TextSpanner #'style = #'solid-line
\override TextSpanner #'thickness = #50
a b c d\startTextSpan
a b c d
a b c d\stopTextSpan
a b\startTextSpan
a b c d
a b c d\stopTextSpan
}

\relative c' {
\override TextSpanner #'style = #'solid-line
\override TextSpanner #'thickness = #100
a b c d\startTextSpan
a b c d
a b c d\stopTextSpan
a b\startTextSpan
a b c d
a b c d\stopTextSpan
}