From https://www.redhat.com/promo/fonts/

"On May 9, 2007, Red Hat announced the public release of these fonts under the trademark LIBERATION at the Red Hat Summit. There are three sets: Sans (a substitute for Arial, Albany, Helvetica, Nimbus Sans L, and Bitstream Vera Sans), Serif (a substitute for Times New Roman, Thorndale, Nimbus Roman, and Bitstream Vera Serif) and Mono (a substitute for Courier New, Cumberland, Courier, Nimbus Mono L, and Bitstream Vera Sans Mono). The fonts are now available for you to install."

I processed a document with these fonts in the latest Prince.
They look good!

Given that the metrics of Liberation Sans are identical to Helvetica, how about aliasing it for the purposes of embedding?

As in bind "Helvetica" to "Liberation Sans" on Linux systems where Helvetica is not installed?

(This can probably be done via a rule in Fontconfig, by the way).